You are seeing the phenomenon of alternative fonts.
What happens is websites are set to have multiple fonts possible for their content.
The "old" font is closer, if not exactly, the primary font. It checks to see if you have it first, and if you do, it uses it.
The "new" font is the secondary font to the "old" font, which it will use if you have it, but not the "old" font.
I think you deleted a font somewhere.