I think the problem is that the cells in C are not really rounded, only their display is limited to some number of decimals. If you have a cell that has a value of 3.14159265358979, when displayed with just two decimals, it appears to be 3.14 though the actual value stored is 0.00159265358979 greater.

This is a disconnect between the spreadsheet world and the world of science where it is necessary to mind significant figures of precision in order to calculate results that most accurately reflect that precision. It also presents problems such as what you are facing though few people worry about a 0.01 difference. It can be confusing if you are presenting numbers that should add to a known figure, say 100.00, and the total comes up as 99.99.

You might want to add another column that rounds the numbers in C to a specified number of digits and then add those. I'm pretty sure that will do the trick.

=round(c2,2)