Word Jumbles

Some of you may have seen this little tidbit in e-mail a few weeks ago when it was making the rounds:

Instructions: Just read the sentence straight through without really
thinking about it.

Acocdrnig to an elgnsih unviesitry sutdy the oredr of letetrs in a
wrod dosen't mttaer, the olny thnig thta's iopmrantt is that the frsit
and lsat ltteer of eevry word is in the crcreot ptoision. The rset can
be jmbueld and one is stlil able to raed the txet wiohtut dclftfuiiy.

What I find strange about my capacity to quickly read that jumble of letters is that I am horrible at real jumbles, i.e. puzzles where you have to discern the word from totally mixed-up letters. I wonder why fixing the first and last letters makes such a huge cognitive difference. Anyone know?