Pack Tactics weird wording?

I've noticed in reading that in both the monster books I have, Monsters of the Multiverse and the 5.5e Monster Manual, that where everywhere else I see "has Advantage on attack rolls", Pack Tactics says, consistently, "has Advantage on an attack roll". Does that wording distinction have any significance? For example, Jackalweres have Multiattack; does the wording mean that only one of its attacks each turn get Advantage?

No, there's no distinction in meaning there; those two phrases mean exactly the same thing. If Pack Tactics were meant to be limited to one attack per turn, it would say that.

Yea every attack roll you make is "an attack roll". 

I think the only significance is that Pack Tactics has a condition that needs to be evaluated per attack. I can't guarantee that other abilities that have conditional advantage per attack follow the same convention, though.