It could be just rounding.
Notice there are no decimal values showing.
You see 23, 46, 45, 39, 349, and 48.
That actually adds to 550, not 540. So it's only two off.
What you might have there is 23.3, 46.2, 44.8, 39.6 and so on ...
The decimal values don't show, but they're included in the total.
So the 548 is the more accurate figure than the 550 is because with 550 you're only adding rounded values.
The figures at the bottom are always the most accurate. :)
Deb, in New Zealand