This is an explanation I have heard. Basically, Kannada madhwas migrated to tamil nadu in various waves spread across different decades or centuries. the families that migrated together became one group. Thus, aravelu, belagandu, aravathu vakkulu, etc were the different groups who migrated at different times.
At some point of time, 60 families must have together migrated to TN, hence that group was called Aravathu vakkulu, meaning 60 houses.
Similarly, velu or velar was some measure of area in olden times during the later vijayanagara empires. There must have a set of families who were donated 6 velars of land by the king. Thus, those families must have been Aravelu.
This is an area which needs closer study, by interested historians and sociologists. And much like most of Indian history, study has been hampered by a lack of written accounts of migrations.
Hope this explanation helped!!!
If anyone can find some more clues, do enlighten us all!!!