The other day I grabbed a different tape measure to the one I usually use.
Goodness gracious, I had lost about 3-4cm! (For those imperialists, that's about 1-1.5 inches, oh except I should say 1 - 1 1/2 inches cos it seems Americans prefer fractions. Those of us in the metric age struggle relatively with fractions, especially when dividing them - much easier to divide 1.375 by 2.5 than 1 3/8s by 2 1/2....)
So I thought the new tape measure must be wrong. I lined up the tape measure with a cardboard ruler that cannot possibly stretch and guess what? My old tape measure is wrong!
Hooray! I am smaller than I thought I was, which explains why I fit into a size smaller clothing than I should. Duh!