Well the settings should be the same for each time, so in theory the image request size should not change from test to test, so there shouldn't be any inconsistency. It might have something to do with race conditions, and if you don't mind it would be great if you could open up a Support Ticket at firstname.lastname@example.org so that we can look into this a bit more and figure out where the inconsistency is coming from! If you do open up a ticket it would be great if you could include a bit more information, with examples from the reports where you have seen this.
(So no, we haven't had any plans until now to look at this issue, but we'd be glad to if you raise a ticket!)
As for the feature request - we do have plans on including mobile Page Speed testing, though it isn't included on any roadmap yet. As for being able to actually specify the pixel density and device width I will forward this suggestion to the developers.
Whenever we have an update regarding this, if it is added to a roadmap etc, we will update this thread!