Real Spellers

I have an extremely dyslexic student who has become fascinated with word sums and how everything goes together.  We were exploring the bound base 'spect' , in part using Word Searcher, and found the word "suspect".  Upon our research we found (in beginner's terms) that it has the prefix 'sus' (a chameleon of 'sub') + spect.  Why, she wonders (and so do I) doesn't the word sum  sus + spect --> susspect?  When mis + spell --> misspell?  As she says, "There is no "drop the s" rule!"