Theme Font Size differs from Online Font Size

Theme font size of articles is set to 12 pt Arial (with Artisteer).

But it SOMETIMES displays online at what looks like 10 pt.

It was displaying correctly online on my computer until about three days ago. Now it displays what looks like 10 pt.

Customer says it has looked like 10 pt on her computer for two or three weeks.

Someone else says it’s 11 pt on their computer. Someone else says it looks like 12 pt to her.

I tried installing a few of the old themes that I know were displaying online correctly from my computer, but they also now display the articles font at 10 pt.

Here’s what it looks like in Artisteer (and how it should look online): http://screencast.com/t/xpTJ3gpe6F

But here’s what the published version looks like when viewed from my computer: http://screencast.com/t/wnnlUYTOW8

Would anyone have any idea where a solution for this might be found??