What is the reasoning on this?

I am frustrated by the spell checking function on computers

     Don't get me wrong.  I use it.  I just don't always understand it.
     For example:  Yesterday in the blog I typed the word electrician.  Only I spelled it eletircian.  Now these were the options offered by the spell check function:  optician, Aleutian, politician, Elysian, elision.   Seriously, do any of those even look like electrician?
     Another time I meant to spell brought.  Instead I spelled brough.  I got as suggestions borough, bro/ugh (seriously the slash was theirs) brought, Abrew and Aubree.  Aubree?  What the hell!
    I typed their as thier.  And their was the first option, but second was Thieu and Thor was third.
(It was the last night of a Roman orgy involving all the gods and several young women.  In the middle of the night, one of the girls got up to use the bathroom and stepped on a sleeping god.  ""Damn you woman!  Can't you see I was sleeping.  I am Thor!" thundered the Roman god.  The girl looked and him and hissed, "You're Thor?  I'm tho Thor I can hardly pith.")
   And when I forgot the e in were, typing wer, I got several alternatives including, wear, wee, weer, weir and we.  Aw was given as a correct spelling for was when I missed the s.
     If you are still reading this, you probably are saying:  "Terry is a terrible speller."  But not true.  I am a terrible typist.  And yes, I failed typing in high school.  Back then you had the old manuals and correcting was not possible.  I had to type 40 words a minute with 5 or less mistakes.  The best I could do was 42 words a minute, but with six mistakes.  The teacher was very apologetic when she told me I was flunking, but I really did not care because I could type.  For someone who was going to win a Pulitzer prize for writing, that was important.
     But I digress.
     Another problem with spell check is it only hits the obvious.  Take this sentence:  Your about two read a sentence witch maid no cents.  Not one spelling mistake.  Huh.