Jaheira for all intents and purposes acts and functions like they're a Good aligned NPC who was simply restricted to a Neutral alignment in the original game for gameplay/rule reasons at the time, being 2nd Edition and all. However she acts, behaves, and talks like someone of a Good alignment, is married to a Good aligned NPC with Khalid, will leave your party if your reputation goes too low, and will get into a fight with the Evil pair of Xzar and Montaron if you leave them in the party together enough.
As for Garrick, he simply doesn't seem to act the same as other Neutral aligned NPCs (besides the previously mentioned Jaheira). Whenever you lose reputation, he comments
"Why did we just do that?"
as to indicate he doesn't approve of negative or evil actions. At the very least, it sounds on par with the tone of what Coran (Chaotic Good) says when you lose reputation. When talking to him the first time and activating the encounter which allows you to get him in your party, you can't get him in your party if you take the evil route of that encounter and kill the merchants, you have to take the good route and side against Silke. From what I have played he will also end up leaving your party if you get too low of a reputation. Finally, if you have a positive reputation he will have passive banter to another Good aligned NPC in your party saying
"You, are an honorable person."
Then someone like Ajantis would generally say something like
"I thank you for your compliment."
Judging by dialogue and behavior, it seems like these are two NPCs that were not given the proper alignments in the original game for one reason or another. And considering there are combinations of stats, skills, and proficiencies a player character wouldn't normally be able to start with (like Kagain's 20 Constitution, Coran's 20 Dexterity and 3 PIPs in Longbows, Dorn being a Half Orc Paladin, Xan being a single class Mage and getting access to a Moonblade sword) I don't think it would be a big deal to have these two NPCs start with what seem to be the proper alignments for their character.