Oh I see, looks like you are sorting by year (which would make sense on an axis)....this is a bit of a workaround, but it will work if you take the table result of the Summary Statistics output and input that into a sorted update cursor (by year), then in a new field calculate the previous record acreage + the current record acreage... make sense?
In other words, use a variable to 'hold' the last record's acreage value + the cummulative acreage to this point in the processing --- so in your example initiating the update cursor to the 1st record, you can fetch the 1st acreage value: 6 (initiating the variable to 0, setvalue for the new 'cummulative value' field to 0)
Iterate over the records, the next record value is for 1978 is 2 (using getvalue) --- add that to your current var value of 6, that's 6 + 2 = 8. setvalue, update the var to 8, go to the next row...
Next row is for 1979 is 5, so 8 + 5 = 13...
Get it? The code is easy to write, see the webhelp examples for cursor processing...post back if you get stuck.
-Wayne