Slightly incorrect formula results

Has anyone else had trouble with Excel being slightly out on its calculations? I've got a spreadsheet where I entered three sets of six weekly figures:

£120.24 £13.00 £9.87
£125.13 £12.00 £8.59
£124.59 £14.00 £7.59
£123.48 £15.00 £6.57
£127.91 £16.00 £6.25
£119.88 £13.00 £7.49

The averages of these figures (which Excel gets right) are:

£123.54 £13.83 £7.73

If you then convert the averages from weekly to monthly (i.e. multiply by 52 and divide by 12), you get:

£535.34 £59.93 £33.50

But Excel gives me these:

£535.33 £59.94 £33.48

I could understand if the answers were consistently wrong (e.g. always 1p down), but as you can see, the first figure is 1p too low, the second figure is 1p too high, and the third figure 2p too low.

Does anyone know what could be causing this? I formatted all three cells as currency with 2 decimal places, and the formula was copied across from one cell to another:

=IF($F$3=$E$6,"N/A",IF($F$3=$E$5,G18,IF($F$3=$E$4,G18*13/12,IF($F$3=$E$3,G18*26/12,IF($F$3=$E$2,G18*52/12,"£0.00")))))

=IF($F$3=$E$6,"N/A",IF($F$3=$E$5,H18,IF($F$3=$E$4,H18*13/12,IF($F$3=$E$3,H18*26/12,IF($F$3=$E$2,H18*52/12,"£0.00")))))

=IF($F$3=$E$6,"N/A",IF($F$3=$E$5,I18,IF($F$3=$E$4,I18*13/12,IF($F$3=$E$3,I18*26/12,IF($F$3=$E$2,I18*52/12,"£0.00")))))