In mathematics, this is how rounding goes:
-2.51 - when you have to round it to the closest tenth, what is next to the tenth digit on the right side - let it be denoted by x (hundredth digit) - has to be 0. You add one to the tenth digit if the hundredth digit is 6≤x≤9 and make the tenth digit even if x=5. If 0≤x4, then the tenth digit remains intact.
In our case the hundredth digit is 1 --> 2.50
Of course you can invent other rules for rounding:
rounding of the last two, or the tenth and the hundredth digits to the higher of the two nearest multiple of five.
2.51 --> 51 (50; 55); 50≤55 --> 2.55
(I have an MSc. in maths)