Very good questions.
Why not go direct into the aircraft via the mister funnel. The mister funnel has a fairly short neck and with the positioning of the tank it means the mister funnel is at a 45 degree angle and is difficult to pour into. It means holding the funnel one handed and the Jerry can with the other. Difficult to do and a risk of spillage.
So why not the mister funnel at the servo. Several reasons for that.
1. I have bad knees and find it very difficult to work at ground level without sitting. As the Cans need to be on the ground to fuel at the servo i find it difficult to squat/bend long enough to fill them.
2. Servos are inherently busy and dirty places. Wind and vehicles driving in and out increase the risk of getting crap into the jerry cans even with the funnel.
3. I keep my funnel and all pouring spouts in a covered container and only bring them out to use them then put them away again, my ute is a work vehicle and is not a particularly sterile environment.
4. At my shed or hanger I have my fuel kit, which includes a 50 year old small chair that my father always used when working his bees. It allows me to sit and be comfortable as I filter my fuel.
5. As already suggested full flow of the 50 mil pouring spout supplies flow at the optimum flow rate for the mister funnel.
6. By draining from one Jerry into the last one drained, and then pouring the last dregs and the bit left in the funnel into my slops can I keep a clean Jerry every time.
7. I have found taking the time to filter my fuel allows me to stop slow down and get my mindset into aviation mode. ie. forget the watch and slow down to my own pace. My work and industry tends to run at breakneck speed so I use this process to slow down and put my mindset into the it does not have to be right now mindset.
8. It is part of my flying experience and I find it relaxing. (Besides I get to sniff the fuel twice).
The point about only purchasing fuel at high volume outlets has come from many years of running vehicles and several very bad fuel experiences. Just a little self taught safety mechanism.
The 10 litre cans rather than 20 litre are simply because I find them easier to handle, and as my filler is under the wing 20 litre cans would be very difficult to manage. I am looking to buy a 5 litre can for those times when I want to fill up the tank but it won't quite take 10 litres.