Do your mates have micra's too...or do they have...for example, pugs?
EG:
Vauxhall Corsa 1.2 Club (2000) has a 44 litre tank
Volkswagon Polo 1.4 S has a 45 litre Tank
Citroen Saxo 1.1 first has a 45 litre Tank
Toyota Starlet S 1.3 has a 45 litre tank
Peugout 206 1.1 independence has a 50 litre tank
...a K11 has a slightly smaller 42.. ?
(...my K12 has 46 so i can get 480 miles, usually, before/when the little light comes on 
)
do they drive as far as you each week?.... (live closer to college/work, work less days etc)
do they drive like their nan?
are you sure they only top up every 2 weeks...is it not possible they sneak the odd tenner in when you're not looking?
does that shed any light?
