Build number confusion

[DKSE]

What's the difference between the 235xx & 28xxx series. I usually see 235xx labeled as 6.5.5 and 28xxx labeled as 6.5.3 but shouldn't the higher build number be the more recent build? Is there a difference between 6.5.3 & 6.5.5? And is seems as if they are being co-developed as a new 235xx usually comes out at the same time as a new 28xxx?