Petrol stations-is a litre really a litre??

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Gidday all,

Did a few miles across town during the week in my 98AU Futura and ending up topping up at the same different petrol station twice.
Now normally I use BP or Shell in my home neighbourhood and usually chuck in $40 when my fuel needle gets to 1/4.This gets me to 3/4.
No difference between these 2-the price is always the bloody same...and so seems the level in my tank.
But during the week I topped up at Gull,again at 1/4 tank,-as that's my "visual" signal,banged in $40, and hey-I've got my needle sitting at midway between 3/4 and Full! I did this twice,and virtually identical result.
I haven't done a full comparison using the DTE meter,but it has to be a good 30km plus for the same money- I'm obviously getting more bang for my buck at Gull.
Which got me to thinking...how accurate is the caliberation on pumps at service stations-and how could there be such a difference??


**To consolidate my point, it is now sunday- and I thought I'd mow my lawns...(sunday bloody sunday!) So I thought I'd turn it into an excercise.
I took both my 6lt jerry cans with me, and went up the road to both BP and Shell.Both had 91 regular at $1.53 per litre.
I put 3lts in one at Shell,and 3lts in the other at BP-drove home and poured them both into my 10lt gardening spray pot.
Well what do you know! BP was 3lts spot on- Shell was 2.90lts- 100ml less.
Isn't that a lot over such a small fill?
Or is it not that simple-

Anyway, I'm filling up at BP now...what a pity I don't have a Gull station around the corner!