Almost *every* device manufacturer does that. And almost every device can be overclocked to some extent. If you look at the specification for a device, you might see it is rated for 25 MHz max over the voltage range 3.0 - 3.6V and temperature range -20 to +85 C. That means the manufacturer has characterised and spot-tested the devices over this entire range. If you run it at room temperature and 3.3V, you will be able to overclock substantially. You'll get even more at optimal conditions (probably close to 3.6V and -20 C) - the fastest speciality PC's have a miniature freezer inside.
You can also consider how manufacturers test and characterise their parts. Devices are made in batches, and random parts from these batches are spot-tested at clock, voltage and temperature extremes (for some types of device, all devices are tested). They are then divided up and marked according to what sales and marketing require. So if a part runs fine at low voltage, high temperature and high speed, it could end up marked as any of these parts depending on the requirements. Thus when you buy a 25 MHz, 105 C part it might run fine at 50 MHz, 20 C.
When testing your own systems, it is not uncommon to overclock to stress the card a little more and check for tighter timing margins.
Of course, if you are making products to sell, you normally want to stick to the limits the manufacturer gives you.