Thanks for that link, Tramfan. One thing it shows (which I hadn't guessed) is that three of the four trams which spend most of their day on T3s start out by doing a round trip on T1, before doing an 'extra' journey (i.e. one which is additional to the standard cycle) to NS as a T2, before finally going on to T3s. So those two trams in the course of their day operate on all three routes, as does tram 'M', which does a one-way journey on all routes, and nothing else.
It's logical, of course, to use trams which have already done a T1 as start-ups for T3s (it saves ten of the fifteen minutes layover at Starr Gate), but the same logic doesn't prevail at 2125/2130.
The tram route cards have been provided through a freedom of information request, and do show that there's a Route F that works the 06:30 Starr Gate to Fleetwood, and 07:40 return (arriving 08:45) and then rather than work the 09:00 Starr Gate to Fleetwood it goes into the Depot and Route A comes out of the Depot to work the 09:00.
Bearing in mind what I've posted above, I would say that
at that time of morning the normal routine is T1 > T2 > T3, but of course it doesn't happen at 0850 because there's an 0825 ex-Fleetwood T3 which is available to work the 0930 ex-NS return. I can't help but think that this is something to do with the odd pattern at 0845/0900, it may have been a mistake.
the main service requires 11 trams, up to 14 can be used throughout the day (from my observations the tram working Route F sometimes doesn't run into the Depot and just becomes Route A).
Have you actually observed tram F running into depot and being replaced by another? I presume you have, perhaps to confirm.
It also means for a brief period in the 21:00 hour there are 13 trams in service at the same time
It's certainly brief (2145 to 2153), and that's if the 2145 picks up service at SG (which it doesn't seem to do) and the '2153' arrives on time (last night it arrived at 2149).
I still cannot fathom why the frequency on the T2 service increases between 21:28 and 22:28.
There's actually only one journey in each direction which could be said to be additional to the standard timetable, 2125 SG - NS and 2158 NS - SG. Tram FF only needs to come out of depot at 2138 to cover for tram D's extended layover at NS (2109 - 2130). Though, like yourself, I can't really see the need for the sudden timetable upset.
The link you've posted is coming up, for me, quite jumbled (that's on my laptop, it isn't coming up at all on my mobile), I don't know whether or not my laptop is doing it, or whether it's a deliberate jumble through being the result of an FOI request.