http://terragalleria.com/blog/2011/03/22/best-iso-for-low-noise-on-canon-5d-mk2/
The results are actually well expected, the immediate intermediate ISO right below each "real" ISO has better (signal-to-)noise performance than the immediate intermediate ISO right above each "real" ISO.
Indeed, I had tested myself with my 5D long time ago and confirmed the same, when no Pentax DSLR had any intermediate ISO values at that time..
However, is there any disadvantage to use those lower noise but faster speed intermediate ISOs? Yes, I am afraid. Indeed, there is one point most people have overlooked, even though the noise advantage has been over-emphasized. That is, the highlight headroom will be less with those particular ISOs, as it is just using the technique of expose to the Right. So the lower noise is actually achieved with the trade-off of highlight headroom! So, just beware! And you should know what you are actually doing if you are selecting this option.
Related:-
The Effects of Real and Digital Exposure Compensations on Exposure