If the original 88 was developed from a naval gun, these weapons originally expressed their calibre in inches. 88mm is very close to 3.5".
Lots of strange metric sizes can be explained by checking the corresponding inch measurement (fractions or decimals).
Rifle calibre 7.62 mm (why not 7.5?) = 0.3 inches
Haven't figured out the background to the .303 caliber though, 7.7 mm is also an odd size.
.22 comes out as 5.59 mm
75 mm could a rounded off value for 76,2 which is 3 inches
likewise for the 155 mm guns, very close to 6.1 inches
the Soviet/Russian 152 mm is the 6 inch guns,
203 mm (why not an even 200??) is 8 inches
Some measurements have been tweaked slightly to fit measuring tools made for the metric system
3.5 inches comes out to 88.9 mm, why not round it up to an even 89 mm? or 90 mm
maybe 88 was easier to remember, looked better, was easier to pronounce or whatever ....