Fill and Sign tool changes font size

I'm having a problem on my Mac using Mohave with the Fill and Sign tool.   It seems that every time I enter some numbers the size of the numbers change.  

I reduce them to the size I want then move on to the next column to enter a number.  However when I enter a number again in the next column,  the size has changed and when I try to reduce I can't get it to be the same size as in the preceding column.  This goes on each time I enter a new number in another column so when I'm finished and look along the form at all the entries they are all assorted sizes, doesn't look to good.

Is this something I'm doing wrong or is it the program.