Deceiving shit like this is done constantly and you probably don't notice it. I'm actually surprised they made it so obvious in this example.

There are dozens of techniques, but if you're curious on one big one, you know those 'New and improved!' or 'New formula!' and so on you sometimes see? That's just to distract you from realizing they dropped 15g of product, kept it at the same price (Or sometimes even raised it a bit), for at the most maybe adding a new cheap fragrance to it, or changing the binder or amount of some ingredients, or even just changing the color and package. Always be suspicious about any well established product that suddenly has something 'new' about it. They can always repeat this same scheme in a few years, and either raise the price or reduce the amount of product you'll get, or if they think they can get away with it, both.

Sometimes they'll just change the package, put some kind of contest on it to attract consumers, and lower the amount of product in it or raise the price, without even bothering with the whole 'new and improved' scheme. Cereal companies occasionally do this.

The worst is when they just remove a few grams of product, and keep the package and price the same size. They could easily turn this around years from now when the product is cheaper, and say '10% more!' or something like that, when they just increased the amount to still less than it originally was. Then they can lower the amount again with very little issue, protest, or anyone really even noticing. This gives them a surprising amount of freedom to adjust the profits at will, allowing them to try and maximize their profits while still manipulating the consumer into thinking they're getting a deal. Snack companies do this dance regularly.

I've only met a handful that have noticed this going on, and they often work in the industry itself or want to. Though recently at least, marketplace, a TV show in Canada, did a whole episode on these deceptive practices.