Font display problem - after uploading changes

[Rik]

After making changes to my web site, I upload the changes and I sometimes get a problem where the font is not displaying properly.

I have attached two examples to show the problem.

In fact, most of the font used is Arial.
With some Trebuchet MS, here and there.

The image called 'visual' is in FireFox on a Mac.
The image called 'printing' is in IE in Windows 7.
In fact, the problem looks to be the same in both browsers.
(The 'visual' image attached - that problem was the same when I looked in IE in Windows 7.)

Of course, if I refresh the pages, then the pages obviously display properly.
Actually, I had to refresh almost every page on the site.

Is there any real reason for this happening?