Let's assume you have a child style of XenForo Default. It has a few small customizations of its own, but the majority of it depends on the parent style (98%).
Let's say you revert 1 template to match the parent.
Now lets say either you've come out with an update, so you upload the xml to overwrite that child.
Doesn't not apply.
That 1 template you reverted earlier, stays reverted and does not adapt the new xml you told it to rewrite it.
This is something that should work (it did in XenForo 1.1.x).
See video as quick example