Was going over adding a steel defender to one of my artificer's Extra's tab and noticed something called an "Improved Steel Defender" (https://www.dndbeyond.com/monsters/improved-steel-defender).

The only difference between this and regular steel defender (https://www.dndbeyond.com/monsters/steel-defender) is that the Improved one has 17 AC over the regular one's 15.

There's nothing in the Artificer Battle Smith subclass that has its steel defender getting improved Armor Class, so why does the Improved Steel Defender exist? Is there a source for it? It just says it's from Basic Rules like the regular one does (which is weird since they ought to be from the Eberron book?).

From the Battle Smith class:

Improved Defender

At 15th level, your Arcane Jolt and steel defender become more powerful:

• The extra damage and the healing of your Arcane Jolt both increase to 4d6.
• Your steel defender gains a +2 bonus to Armor Class.
• Whenever your steel defender uses its Deflect Attack, the attacker takes force damage equal to 1d4 + your Intelligence modifier.

They use the Improved Steel Defender extra to make this change because of various technical limitations stopping them from boosting the normal one.

Ah okay thanks!