Why are foods often packaged in weird measurements?
Quite often, it's because a manufacturer prefers to decrease the size of a packet than to increase its price.  Customers notice when the price goes up but don't tend to notice the product getting smaller.

For example, jam in the UK used to be sold in jars containing one pound, which became the equivalent 454 grams with metrication. But, more recently, that's shrunk to 400g and even 350g in some cases. Similarly, according to Wikipedia, Yorkie bars used to be 70g, which then became 64.5g, 61g, 55g and now 46g; packets of crisps that used to be 30g are now often 27g (10% smaller) or even less.