Is the ability of reading NFC-tags in smartphones specified?
From experience I know that NFC performance varies very much among different smartphone models.

And there is no reason to assume that they use only one type of NFC chip; and even if they did there are other very important factors that can make a diference:


RF PA (if not included in NFC chip)
antenna design (and RF relevant design of the whole device)
matching network


Also the max. distance for NFC not only depends on the terminal side (i.e. smartphone that provides power via RF) but also on the type of tag or NFC card that consumes the power. A NFC smart card that perform cryptographic operation needs much more power than a dumb NFC tag that just answers a few fixed bytes.