I have the MSO6012A with the 8 MSa option. When I set the scope for 100 ms/div (1 second full width), the scope reports a sample rate of 2 MSa/s for continuous trigger or 4 MSa/s for single trigger mode. That would be a total of 4 MSa. Where are the other 4 MSa?
Using peak detect acquisition mode
Using more than 2 out of 4 analogue channels where present (note that for max memory on 2 channels you need to use 1+3, not 1+2), or >1 on 2-channel scopes, I think.
Not using single-shot mode - in normal/auto trig modes, the memory is double-buffered so you only get half for each acquisition.