The distributive property of multiplication states that X*(A+B+C+D) = XA+XB+XC+XD, so in pure math, there should be no difference.
However, as Virg said, people tend to round. When calculating tip, I tend to do a fair amount of rounding, but the last thing I do is always round up to the next whole dollar.
For example, a $49.12 tab becomes $49. 10% of $49 is $4.90 (which is about $5). Half again is $7.50. Finally, round $7.50 up to the next whole dollar, and my final tip is $8.
In that example, I'm thinking the tip will be about 15%, but in reality due to rounding each step to make the mental math easier, the final tip is a bit over 16%.