#9 in Belgian beer bottle seems to work

[ThePrisoner]

Not sure why but it is almost impossible to find Belgian beer corks in Canada and it's too expensive to ship from the US.
I know general consensus on the forum seems to be that a Belgian beer bottle needs a 25.5mm cork and a #9 wine won't work but I measured the opening of a wine bottle to be 20mm and the unibroue bottles I have are also 20mm.
What am I missing? Why does a 25.5 mm need to be used if the opening is the same?
Pic attached of a #9 that I put in with a floor corker.

 

[mac_1103]

I think the point is that #9 corks fit too easily. They won't "mushroom" and since they don't get compressed as much they may not hold carbonation long term.