Phil Gollin
You ask how so similar a thing can occur in two separate places but with a time difference between them.
I have to say that I can't see that there is much difference in reality, six or seven seconds can be accounted for by a slightly different amount of ice (in some form) being present and the 'detaching' behaviour of that accumulated ice by the slightly different fuel flows noted in each engine.
Since this is the only case ever seen of this nature, there are no statistics with which to determine whether the similarity seen is a coincidence or not.
Let's face it, it has been shown that ice is the only remaining possibility, and how else can such a restriction occur other than that some form of solidified ice arrives at a constricted area in high concentration, overwhelming the ability of the fluid part of the flow to pass at sufficient rate.