You can think of umask as somewhat opposite to permission.

By default Ubuntu uses the base permission of 777 for a directory and 666 for a file. By deducting the umask value from the base permission you get the applied permission of a newly created file/directory.

In your case, you have given the umask as 755. Now as the base permission for directories is always 777, what the applied permission we get:

777 - 755 = 022 i.e. ----w--w-

To get your desired permission (755), you need to put the umask as 022.