Why such a large jump in progression

Why at the end of base 1 if I am at a SS progression level of 4.4 is my fist w/o of base2 a SS 5.9?

Usually, once you get into the first week of SSB2, the plan will make adaptations to workouts to tie in with your current levels. If you have an FTP test at the beginning of SSB2, it’s likely your levels will change anyway.