Picture strange behaviour rounding Real numbers

Hello everyone…

I need help with a strange behavior of the picture that I have defined.
My data type is Real and I use the picture @ N-16.3, when the value of my variable is 0.9998, it automatically rounds it to 1 in the textbox. I only need precision of 4 decimal places but to achieve it I must use @ N-16.5.
The question would be someone knows why it is automatically rounded? Is there a way not to do it?

As always a thousand thanks in advance