No, the Save DC is based on the Bison's Strength. For example, if someone were to cast Bull's Strength on the Bison, the DC for his ability would increase by 2.
Essentially, these notations are used when advancing a creature by Hit Dice (and also when advancing by class, but less so, since class levels don't result in size changes, and thus result in less stat modification). Thus, if the bison's Strength goes up, so does the save DC of the trample ability.
It means: "This DC was calculated as 10 + HD/2 + Str bonus = 10 + 2 + 6 = 18. If the Strength changes for any reason, you need to re-do that calculation on your own."
In 3.5 every monster ability DC has a "This save is ____ based" note at the end, for the same purpose.