Na angielskiej wikipedii znalazłem takie coś, nie za bardzo to rozumiem
Round half to even
A tie-breaking rule without positive/negative bias and without bias toward/away from zero is round half to even. By this convention, if the fractional part of x is 0.5, then y is the even integer nearest to x. Thus, for example, +23.5 becomes +24, as does +24.5; while −23.5 becomes −24, as does −24.5. This function minimizes the expected error when summing over rounded figures, even when the inputs are mostly positive or mostly negative.
This variant of the round-to-nearest method is also called convergent rounding, statistician's rounding, Dutch rounding, Gaussian rounding, odd–even rounding,[6] or bankers' rounding.
This is the default rounding mode used in IEEE 754 floating-point operations (see also Nearest integer function), and the more sophisticated mode[clarification needed] used when rounding to significant figures.
By eliminating bias, repeated rounded addition or subtraction of independent numbers will give a result with an error that tends to grow in proportion to the square root of the number of operations rather than linearly. See random walk for more.
However, this rule distorts the distribution by increasing the probability of evens relative to odds. Typically this is less important than the biases that are eliminated by this method.