Bonus words?

Can somebody explain to me why some words become "Bonus Words" and don't count in your list? The explanation given in the how to play section make absolutely no sense to me.

Thanks in advance to anyone that can help.

Bonus words are not on the list because they are not words made from the 7 letter word - for example - if you make SKATE - but in the main word there is only 1 S - so if you put the S on the end of SKATE and get SKATES - that is the bonus word - you cannot make SKATES if you only have 1 S
Does that make any sense.

Cheers Babs