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A true ounce is 28.5 grams. A Mexican Ounce is 25 grams (not 24 grams as incorrectly defined above). The reason the measurement is shorted is because it is easier to divide and multiply by 25 rather than 28.5 when converting between imperial and metric measurements. That way when it is split from larger quantities like kilos or pounds, it divides into even smaller quantities and there is no remainder.

For example:

1lb = 16oz = 400g (true measurement of 456g)
1kg = 40oz (true measurement of 35.2g)
Johnny: I went to Hector for an ounce and when I weighed it out it was 25g.

Robbie: Damn! That's only a Mexican Ounce. Last time we buy from that dude.
by lx5spd November 12, 2016
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