For your locale settings, , is the decimal separator, so that's what you need to use to enter a number. If you use a period, the input will be interpreted as something other than a number; what that is depends on the content.
If what you enter could be a valid date, a period is being interpreted as a date separator. Excel thinks 1.25 is a date (January 25, 2018), which is represented internally as 43125 (the number of days since January 0, 1900).
Since you formatted the cell as a number rather than a date, you're seeing the numerical representation of the date. The default decimal places is set to two, giving you the zeros at the end. It's pure coincidence that the "125" pattern is repeated in the day count representing that date.
Both 1.01 and 1.9 could be dates, so the same thing happens to them. But 1.99 cannot be a date, so the literal value, as entered, is stored as text.
