I wouldn't worry too much about it - it is probably a coincidence. Britannia do have a habit of reallocating flight numbers from scratch each season and so it's not necessarily a foregone conclusion that a Newcastle-Mahon on Saturdays in Summer 2005 will have the same flight number as that in Summer 2004.
Brit's summer season flight numbering system (BY1A/B operates on Monday, BY2A/B Tuesday and so on, with BY8, BY15, BY22 all being the subsequent Monday flights) lends itself to numbers sticking with days of the week rather than individual flights.