Two identical phones give different read distances
Same model does not mean same conditions. Strip both to a bare phone, same tag, same spot, same duration, and retest. The difference is almost always something attached to one of them, and manufacturing tolerance accounts for far less than people assume.
Strip both phones
Cases off, mounts off, cards out. Then test both with the same tag, in the same position, held for the same time.
If they now behave the same, the difference was something you removed, and you have your answer in a minute.
If they still differ, put one variable back at a time until the difference reappears. Changing several at once makes the result meaningless.
Use a small tag for this rather than a card. A tag with little margin exaggerates differences that a card would hide.
What is usually attached
A thicker case on one phone. Even a few millimetres matter on a small tag.
A metal plate for a magnetic car mount, which stops reading outright rather than reducing it.
A card in a wallet case, sitting exactly between the coil and the tag, absorbing field and answering the reader itself.
A ring, a popsocket, or a grip with metal parts, at whatever spot it occupies.
A screen protector is not a candidate, since the coil is on the back.
Settings that differ
A power saving mode active on one device, which can restrict or disable NFC depending on the manufacturer.
NFC disabled outright on one, which is easy to overlook when the phone reads sometimes because someone opened an application.
Different software versions, if one phone has updated and the other has not.
Check these before concluding anything about the hardware, because all three are free to check and common.
What is left after that
Manufacturing tolerance is real and small. Two bare phones of the same model, in the same state, generally behave close enough that you would not notice in normal use.
If a genuine difference survives every check above, one phone may have been damaged, most likely by a drop or by water.
At that point the useful question is not why they differ but whether both are adequate for the deployment. Test both against the worst case tag and mounting rather than against each other.
And if one is marginal, treat it as your reference device. A deployment that satisfies the weaker phone satisfies the other one automatically.
Frequently asked questions
Can two phones of the same model really differ?
Slightly, from manufacturing tolerance. In practice the difference is almost always a case, an accessory or a setting rather than the hardware.
Does a screen protector affect NFC?
No. The coil is on the back of the phone, so anything on the screen is irrelevant to tag reading.
Why does one phone need a longer tap?
Usually a thicker case or a weaker coupling from an accessory. Strip both phones and compare before looking further.
Should I test on the better or the worse phone?
The worse one. A deployment that works on the weakest device in your population works everywhere above it.