Different wikis are configured to display mapframe maps differently. On Wikivoyage, for example, mapframe embedded maps are dynamic. While on most Wikipedias, the embedded maps are static (presumably to speed load time or spare servers?). A user has noticed that the static maps don't display the scale gauge that is shown on the dynamic maps.
The static maps correctly don't show zoom controls, since you can't zoom the static maps. But I can't think of a reason why the scale key, a useful informational item, should not display on the static map. (Note that when dynamic mapframe maps are very small—the lower limit seems to be around 200 px—the scale gauge drops out, which seems correct.)
You can clearly see this behavior on these two map test pages: testwiki, which is configured for static maps, shows no scale markers. While test2 wiki, which is configured for dynamic mapframes, does show the scale.
If anyone knows a reason why this omission is a feature and not a bug, please speak up. Otherwise, I'm putting this into Ready for Pickup.
With gauge (on test2)
Without (on test)