Different monospace fonts are used for inline code snippets (using a single backtick `) and for block code snippets (using three backticks ```).

This is most visible with characters like '0' (zero) or 'l' (lowercase L), which tend to be the most visible difference between monospace fonts.

It looks like blocks use the font defined in Phabricator preferences (in my case, DejaVu Sans Mono) while inlines use the default system monospace font (in my case, Consolas). Even with the default settings, the font size is different.

Example: l0

l0