My understanding it is simply due to the naming conventions of other groups of Angles and Saxons and thier locations on Great Britain.

For example you had a group in Mercia known as middle Angles and to the south and west you have the South and West Saxons so when you take into account these names it really isn't that strange. So really it has nothing to do with the location of Anglia at all and is all to do with thier position in relation of other Saxons and Angles on Great Britain.