Adds too much tax

I’ve noticed a strange thing.

Example:

Item Description	Qty	Sub-Total  	Tax %	Tax Amt
Product	                  1	SEK192.00	25.00	SEK48.00

Sub-Total	                SEK192.00	        SEK49.00

Shipping (Shipping Rates)	SEK50.00
Total Order Charges	        SEK242.00 (excl.tax)    SEK291.00 (incl.tax)

Why does the calculation add an extra 1 SEK tax on the sub-total calculation?
192 + 25% = 240
240 + 50 = 290
AND NOT 291!