First, a little about row and column measurement. When you click and hold the mouse pointer on a row or column divider line (e.g. the line between the 1 and 2 or between the A and B), Excel will show, using row height as an example, Height: 12.75(17 pixels). Row heights in MSExcel are measured in points (pixels) and Column widths are measured in characters(pixels). While the points/character measurements are shown first, Excel actually translates everything into pixels.
There are approximately 96 pixels to the inch (2.5 cm) — and Excel does not do partial pixels….and therein lies the problem.
Close your eyes and jump to the next paragraph if you don’t like seeing the math thing:
Sample Gauge: 36sts over 4 inches (10cm)
36 / 4 = 9 sts per inch (2.5cm)
since there are 96 pixels / inch (2.5cm), 96 / 9 = 10.7 (the width in pixels of each stitch)
You can open your eyes now 😉
So, your gauge requires that each stitch (Excel column) be 10.7 pixels wide. Since Excel does not do partial pixels, each of your Excel graph paper stitches (cells) is off by 0.3 pixels.
At maximum, the gauge could be off by as much as 0.5 pixels. On a graph for a sweater that is 120 stitches across the front and 120 stitches across the back, this discrepancy results in an error of over 1 inch (2.5cm) in the total garment circumference.
Depending on your garment design, it may be close enough for charting designs, but if you want to use it for actual stitch counts, decreases, etc. there’s enough error here that it’s only really useful if your gauge divides out to be an even pixel number.