Trying to reconcile 2 Bitcoin numbers regarding private keys

Hello

I have been trying all my life to be good in maths but my brain lacks some abilities. Can you help me find where I am missing the point ?

On one hand, with base 58 on 51 characters, there could be up to 58^51 possibilities for the encoded private keys. This is 8.6E89.

On the other hand, there could be 2^256 private keys based on number of bits. This is 1.15E77, much less.

Where is the mistake ?

Thx

EazyE