Row alignment

I already read a topic on this but with no solution suggested. My spreadsheet shows the lines misaligned in the left section (the ones which numbers the rows) against the rest of the document. If I manually change the row height it's fixed, but if I move to a different sheet and then I switch back to the first one, the problem happens again.

I tried with Camino, Safari, Firefox (both under PC and Mac Os X 10.5), any idea? I could also share the file with you if you tell me which user would you use :)