Scott is correct. Rounded letters are usually slightly higher than their square counterparts. This helps create an illusion that the letters are actually the same height when side by side forming words. If all the letters were indeed the exact same height the letters that are curved at the top and bottom will actually appear to be smaller in height due to the eye's limitation.
This becomes quite apparent in the case of the words formed here as the letters fail to delude the eye since each word only has one rounded letter each.
Avenir does tend to take the effect to the extreme and perhaps a screen view setting could reduce the noticeability slightly but not fully.
If the letters need to be the exact same size best thing to do it to use an actual art program such as Adobe Illustrator and manually set the height of each letter overtop your photo after the fonts have been converted to outlines.