RIP Ellie Stumbleduck 

Despite having a proficiency bonus of +2, this creature has +4 on all skills.

Quote from golw >>

Despite having a proficiency bonus of +2, this creature has +4 on all skills.

 Dexterety + Proficiency = 4

Quote from SimonEttlin >>

Quote from golw >>

Despite having a proficiency bonus of +2, this creature has +4 on all skills.

 Dexterety + Proficiency = 4

 I think he's referring to the skills, not the attack bonus: 

Skills Nature +4, Perception +5, Stealth +6, Survival +5

Wisdom is +1 yet perception is +5 suggesting a proficiency bonus of +4. I believe this is because scouts are NPC rogues with "expertise" (double proficiency bonus). 

Give or take, this is more or less true. They do have Expertise, a feature which is also visible on things like Giant Eagles. You'll also notice that a majority of their Stats & Skills are tied either to their CR or their Hit Dice (such as a Sneak Attack on a Rogue NPC).

One bit of correction however, these are basically Rangers, not Rogues, however these specific Rangers chose to make a living from being a Ranger, thus their Mastery of those specific Skills is directly tied to their Paycheck, motivating them to have the dedication to their Skills akin to a Rogue, as Spell Slots can run out, but those skilled in Tracking can follow their Prey for Days.

They're Mercenary Rangers basically, but they are lacking the Magical connection and relying more upon their Skills, thus they traded Spell Slots for Expertise.

While some people above have said that the abnormal stats are a result of expertise I'd say it's actually the Natural Explorer feature. The Natural Explorer feature allows a ranger to have expertise in INT and WIS checks related to their favored terrains.

A scout has a passive Perception score of 15, but also advantage on Perception checks that rely on hearing or sight, which is the vast majority of Perception checks. Advantage adds 5  to a passive score, so scouts effectively have passive Perception 20 most of the time.

Quote from mdrichey >>

A scout has a passive Perception score of 15, but also advantage on Perception checks that rely on hearing or sight, which is the vast majority of Perception checks. Advantage adds 5  to a passive score, so scouts effectively have passive Perception 20 most of the time.

 I would assume that is already factored in. I would use the passive perception the stat block indicates.

The skill bonuses are accurate for a L3 Scout Rogue: she has double proficiency bonuses (2 * +2) in those 4 skills (2 from Expertise, 2 from the archetype's Survivalist feature).

The 3 hit dice are also consistent with the idea of her being a L3 character.

I’m convinced an NPC starts with 2 hit dice in place of maximized hit point at 1st level. The acolyte is a 1st level spellcaster and they has 2d8 hd (spellcaster level too, is often unrelated with their hit dice, even with this theory). Proficiency discussion is all right but Jeremy Crawford said that an NPC can have any number for their skill bonus.

Most likely has expertise cuz' it's kinda like a Rogue.

Quote from Indrafatmoko >>

I’m convinced an NPC starts with 2 hit dice in place of maximized hit point at 1st level. The acolyte is a 1st level spellcaster and they has 2d8 hd (spellcaster level too, is often unrelated with their hit dice, even with this theory). Proficiency discussion is all right but Jeremy Crawford said that an NPC can have any number for their skill bonus.

 

Well the player levelling rules are designed for PCs, not npc statblocks. But if you were to start treating all humanoid npcs as if the levelling rules applied to them, then that's a fair way to go about it. Less finicky.

They're neither rangers nor rogues, though they could be anything in between. Two attacks and no sneak attack does make them closer to rangers though.

Wonderful option for the Sidekick Expert. Good stats, nice benefits to perception, and the ability to use multiattack to give the expert more attacks.

My group has one as a warrior and yeah, they seem to just make really good sidekicks in general

They're also great for when you want actually threatening bandits without overkilling your party with archers, thugs, or veterans.

Good expert sidekick

Fun fact: the Scout's actual Passive Perception is 20. Since they have advantage on Perception checks that involve hearing or sight (which is most Perception checks), that amounts to a +5.