That’s the difference between a hex key and a npub. There are different requirements but each have certain characters that can’t be used so it can be difficult to come up with a good one.

Also keep in mind difficulty is exponential– 4 or 5 character keys are do-able but every extra character doubles the expected time to mine.

I think I used rana btw, you can choose to mine either hex or npub formats, also you can mine multiple keys at once which is faster.