Guys....I'm trying to wrap my brain around this and make the right decision on sizing. Any help would be greatly appreciated.

Current ride
Colnago C50
Reach 395
Stack 546
Headtube 158


Desired bike
Which size is correct for me ?
Parlee Z5 Medium
Reach 383
Stack 545
Headtube 140

Parlee Z5 Medium Large
Reach 388
Stack 564
Headtube 158

Based on the reach and stack, it seems as though I should be riding the medium Parlee.....however, I'm confused by the sharp contrast of headtubes on the C50 at 158 and medium Parlee at 140. Can this difference be explained entirely due to the C50 having a much slacker headtube ?

Can someone explain this to me ....am I thinking correctly ?

Thanks