I've had a second thought about this. Your Allexanders may have spelt the name with the second L on purpose though if they did so why did Alexanders elsewhere not use a second L. If they were traders it is eye catching and people would tend to stop at their store to pass comment on the mis - spelling.
An alternative could possibly be a marriage with a family with two Ls eg Allen - instead of a hyphen they rolled the two names into one and dropped the " en ".
And then maybe they just wanted to hide. Being Alexander would make them easy to find in a directory. However being Allexander would make it slightly more difficult to find them as fewer people would tend to turn to the double L version of the name.
These are only theories but at least they may be worth considering as a clerical error does not have the same appeal.