Got a tricky error in my Excel (2004 for Mac, Version 11.2 (050714)

The difference between the two figures:

a1 6833.40
a2 6828.66

Important note: both figures were manually entered exactly as above, no
further (hidden) digits.

Is correctly calculated by Excel as 4.74 in cell a3. So far, so good.

However, when increasing the number of digits for a3 to the max of 30, then
Excel returns the result:

4.739999999999780000000000000000,

Why is this?

Thanks, Michel