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.