Why variant has to have default UOM same as that of Template

When I change the default UOM of a variant say from KG to meter, while the default UOM of Template (from which the variant is derived) is KG, I see a pop-up on saving that
Default Unit of Measure for Variant must be same as in Template.

which is clearly in-line with code at https://github.com/frappe/erpnext/blob/4055cc76a73037fce8afe74a1aadbb4b3e16f6a9/erpnext/stock/doctype/item/item.py#L943

But what is the reason for this restriction?

Tagging for reach.
@rmehta @revant_one @peterg