html fonts display differently

On a web page,letters are generated on the web for parents where student letters need to be generated.
The html font on the letters are setup to be either:
1. Arial 10 or
2. arial, san serif 10.

The letters are display ok, but the font on the letters look different depending upon the font type that is specified on each unique html line.

I know that Arial 10 ahould appear on all the lines, however the fonts look different from where Arial 10 versus arial, san serif 10 is displayed.

As far as I know, on the lines that say 'arial, san serif 10' arial not being picked up? Can you tell me why? I always thought that if the first font was not available
on a particular printer, the second font will be picked up?

Thus can you show me what I can do to solve my problem?