Glad you enjoyed the free airshow over MIRSI (it's spelt that way - with an I - not that it matters greatly
)
I think holding aircraft do deviate slightly all the time, probably due to such factors as their speed, the met conditions, on board aicraft systems - all that sort of stuff.
Today there was an Aer Arann aircraft that was irregularly holding over MIRSI too, as the crew made their mind up whether to divert to Liverpool or not. You may have witnessed that in it's elongated pattern.