I don't know how much clearer you need it explained - the C700 has a lower clock speed than the C750/C860.
If you own a C700, then Default(PXA250) is normal, and everything above that is overclocked.
If you own a C750/C860, the Default(PXA255) is normal, and only "Overclocked" is faster. All the other options are slower than normal.
"Normal" is only called that because maslovsky couldn't think of a better term - NOT because it is any sort of default.