Assignment of card UID is handled by NXP during manufacturing of the Chip before the chips are inserted into the cards.
As a supplier of Mifare cards we have no control over what UID numbers are on the cards chip. This is the case for most suppliers of Mifare cards.
When you inspect a card on your system the UID may show as 10 digit decimal UID value but it is not the case that a mifare UID viewed in decimal always has 10 digits.
The actual mifare UID is a 4 or 7 Byte hexadecimal value (e.g 4E DC F0 81 on a 4 Byte card, see attached screen shot from the NXP tag info android app),
The decimal value (e.g 1567935364 )is a conversion of the true hexadecimal value.
This number can vary in length so if we supply a box of cards some UID's may be longer than others when viewed as a decimal value.
An simple analogy would be a Police car that reads number plates but it can only read a plate in the new format eg MA67 SED and not the old numbering scheme of Y999 YYY or Personalised plates that are still valid.
The mifare Classic EV1 cards we supply have 4 byte UIDs’ and we can source 7 Byte UID if required.
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