The ones with the very small nonces are likely to be proof-of-stake blocks. Don't forget that these are also in the mix and will account for the short block time you are referring to.
Oh! That explains a lot of my confusion. The proof-of-stake implementation details are especially vague. How are those mined? (Who does the (obviously simple) computation for it and gets rewarded?) Why is P-O-S a 10 second block instead of for example folding it as an extra parallel step into hash and prime mining? The P-O-S blocks, because of their 10-15 second duration, really dominate the Nexus blockchain storage size and bandwidth.
POS blocks don't have a 10-15 second duration. They happen every 60 seconds:
http://www.nexusearth.com/specs.htmlWhen you look at a mix of POS (150 seconds) and POW (60 seconds) blocks, the average time between any blocks should be about 42 seconds. So it's no surprise that you see many blocks with 10-15 second duration, just like you see frequent Bitcoin blocks with 3 minute duration.
I don't know the answer to your design question. Vid. would be the authoritative source on the reason behind it. The POS blocks are also tied in with the trust keys, which is probably why they are independent, but I don't know.