It is due to what data type it is stored as, and how they can be used.Let’s sat we have the number 21
An integer (or float) is a data type you can do math with (as in 21.5+20 will equal 41.5).
Textboxes only accepts texts (letter-characters) which doesnt have this function. So for a number to be recognized as text, it must convert the munber into the characters that is is made of, and tell it “Hey, you must behave like letters”.
So now, they are not being recognized as numbers that you can do math with, but insted like a character such as A,d, # and ?. So if you would add to Texts (or more precisely strings, which also consists of characters) as before you would get (21.5 + 20 equals 21.520) as in how you would lay words next to each other.
This, the textbox can understand, and translate to something it can show.
This process is what happen when you convert from integer/float to text/string.
Hope it makes sense 