meld says it uses "monospace regular", but shows a variable-width font

I'm using meld 3.16.4 on a CentOS 7.x machine (yes, woe is me). Now, the font meld uses for the text of the files it compares is a variable-width font, but - the preferences dialog says the font is "Monospace regular". I know what Monospace regular looks like on other systems - and it's not what I'm seeing from meld; the font I see is variable-space and sans serif. What's going on and how can I fix this?