Default number of decimal digits displayed is six. There are many ways to display more, for example:

NumberForm[29204.24 - 5840.85, 7]

yields

23363.39

Approximate numbers (anything with a decimal point) use an internal binary representation. This means that internally, approximate decimal inputs are rounded to the nearest binary floating point number, so results may not exactly reflect an exact decimal calculation. For exact calculation, use rational numbers:

2920424/100 - 584085/100

yields

2336339/100