When my son was a toddler (he is now almost 7 so my memory is vague), I remember a conversation with his dr. about blood pressue. I was concerned because we would get these wildly different readings depending on whether his b.p. was taken for an endo appt or a gen. ped. appt.  Sometimes they were very high, then a couple of days later it would be normal.

His dr. said the readings were impacted heavily by his activity level during the attempted reading, what type of reading they were doing, and the size of the armband. I think it especially mattered if they checked manually or with the automated machine. (I say automated for lack of a better word, it does have a name) 

Hope that helps.