Why does Excel round correctly sometimes but not all the time when using the
"decrease decimal" function?

Take for example the number - 6.464640
When you decrease the last digit you get - 6.46464
Excel rounds correctly, 0 is less than or equal to 5 so the 4 stays as is

When you decrease the next digit - 6.4646
Excel rounds correctly, 4 is less than or equal to 5 so the 6 stays as is

When you decrease the next digit - 6.465
Excel rounds correctly, 6 is greater than or equal to 5 so the 4 rounds to 5

When you decrease the next digit - 6.46
Excel does not round correctly, 5 is greater than or equal to 5 so the 6
should round to 7 but does not

Why is that please? Thanks! Barb