Once upon a time in the air racing world there was a ban on race numbers ending in zero (30, 40, 50, etc.) because a pylon judge might call a pylon cut on "number 30", and it might be heard as "number 13". (see note)
To avoid this ambiguity the above rule was applied. It sounds to me like ATC communications might be equally vulnerable.
Note: This rule was not in force in the 1930s; Mister Mulligan was race no. 40.