Postal weights and excess charges

I'll assume that those who frequently use the postal service have got used to the new charging system based both on weight and size so I'll skip the explanations and get to the meat.

If you post something without the correct charge the post office charge the difference between the stamped value and the second-class value then add £1 to it for the hassle of keeping it at the sorting office and having to fill out one of those cards.

Okay so far so fine until someone got a card through. The charge was £1.15 so using the printed sheet I keep up it's easy to spot this is a large letter with a 66p first class stamp that's over 100g in weight which bumps it to an 81p second class. Okay an easy mistake to make, one would think.

I get a hold of the unopened letter and weigh it at 127g, okay that's not a easy mistake to make. 105g or such I could understand, but 27g over is a bit much. So how did the sender come to put only 100g or less payment on this letter? They took it their post office and had it assessed there.

So either the letter gained 27g in the post or someone's scales aren't right. Hmm I wonder how much profit there is in that £1 charge?