Weretigers (and all lyncathropes; Monster Manual p. 210) have the Shapechanger trait:
The weretiger can use its action to polymorph into a tiger-humanoid hybrid or into a tiger, or back into its true form, which is humanoid. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed.
(Similar language can be found in MM for all lycanthrope types, not just weretigers.)
So if a weretiger PC in humanoid form wears studded leather armor and has a dexterity score of 16, say, it will have an AC of 15 (12 + DEX mod). If that weretiger polymorphs into its tiger form, the armor isn’t transformed with it, so does it keep its AC with the armor (“its statistics… are the same in each form”) or does it lose the AC provided by the armor (since “any equipment it is wearing… isn’t transformed”)?
Does a lycanthrope's AC change when shapechanged?
To clarify, my confusion lies with what seems to me to be a contradiction. Either the lyncanthrope in animal form retains the AC it had in humanoid form (because its statistics are the same for each form), or its AC is possibly lowered (because its armor doesn't transform with it and I assume said armor no longer fits so its AC doesn't benefit from that armor). To stick with the example above, say the weretiger PC clad in studded leather armor and in humanoid form with that AC of 15 transforms into a tiger. The armor is no longer effective—it doesn't fit the tiger's shape or size and falls (or bursts) off. Does the tiger have an AC of 15 (its statistics are the same) or 13 (because it loses the armor's bonus to AC)?
Note: Since this question is in regards to PC lyncanthropes, not monsters or NPCs, the AC listed in the stat block for each type of lycanthrope doesn't satisfy my question.
Best Answer
The AC changes, to 13
In their human form, the PC weretiger with 16 DEX wears the studded leather armor, giving them 12 + 3 (= 15) AC.
They polymorph into their tiger form.
As you note, the armor does not transform to fit them, so they do not benefit from the human-shaped armor. It is the DM's discretion whether the armor is destroyed, damaged, or just hangs off the weretiger's new form uselessly after the transformation.
Since they are not benefiting from the armor any more, they can no longer use the AC calculation (12 + DEXMOD) provided by the armor. As Solias notes, in the Player Characters as Lycanthropes box on MM page 207, AC modifications in different forms are noted for other types of lycanthropes, but not for weretigers, so their AC is now simply the basic 10 + DEXMOD = 13.
Equipment is not a statistic
Your confusion seems to be centered around the following passage:
And this is true.
If the weretiger were wearing a Ring of Fire Resistance which it took off and dropped exactly as it transformed, it would not still have resistance to fire in its tiger form, despite "resistances" being a statistic. This is because fire resistance is not a statistic of the PC, but something provided by the ring to the PC.
The same is true for the AC calculation (12 + DEXMOD) provided by the studded leather armor - it is not a statistic of the PC, but something provided by the armor to the PC.
Jeremy Crawford has tweeted to clarify that equipment is not an inherent part of monsters in the context of a similar question around the Shapechange spell - the same is true of PCs.