Thanks for reporting this.
Epeli, your calculations are off, because you forgot that the blueprint must first take into account the base requirements of the optional component before adding difficulty modifier. E.g. the second calculation in your post should read: 62 + 6.2 = 68, working as intended.
But you were close to pinpointing the problem. It actually occurs when there are multiple optional components with diff mods and are placed in such way that the second component's base difficulty is higher than the total blueprint difficulty including the first optional components diff mod (in OP that's the barrel retractor with 70 mech requirement). This swallows up the bonus difficulty of the first component in the calculation leaving the calculation with only the second diff mod. When you place them the other way around this doesn't happen and hence the difference in requirements.
I hope I managed to clear it up. Anyway, it's fixed now.