Signs have 12 pixels from top to bottom. However, there are another 4 block pixels above and below the sign, which total to 20, rather than the 16 that would be expected. Also, the sign is 24 pixels wide, rather than the expected 16.

This (alongside several other sign issues such as MC-2836, MC-28561, MC-157547 and MC-195472) could be fixed by using block models for signs rather than entity rendering, and adjusting the proportions for the signs.