Perhaps what was sent was two things: a form with various parameters (name, birth date, amount of credit, etc), and the contract.
The bank then only noticed the changes in the form (since they have to input it to their systems anyway), and didn't notice the changes to the contract (since it's a contract of adhesion, and thus not supposed to be modified).
Good points. Sucks that there aren't more facts available.
Contract of adhesion: Wow very interesting point. If Tinkov's offer is construed as a unilateral offer, acceptance was only possible by performance (signing and returning the offer).
If Tinkov gave him the unlimited line of credit that he asked for, then didn't they by their conduct consent to his variation of the terms?
https://www.rt.com/business/tinkoff-bank-agarkov-credit-299/ <- says here they approved his unlimited line of credit