Pure Brute Force has Higher Hash Rate Than Fixed Chars

[pluna]

When I run hashcat with -a 3 -m 12700 HASH.hash ?a?a?a?a?a?a?a?a (len 8), I get about 280MH/s. However, when running the same with some chars prepended (eg: abcde?a?a?a?a?a?a?a?a {len13}), my hash rate goes down to about 200MH/s.

Why is this? Is there a way I can run it with the higher has rate but also prepend my chars?

Thanks