Here's why:
* Tire Contact Patch: When a tire is mounted on a vehicle, a portion of the tire's surface is in contact with the road (the contact patch). The vehicle's weight presses down on this contact patch, compressing the tire's sidewalls and increasing the air pressure slightly. This isn't a massive change, but it's measurable.
* Removal of Load: When you remove the tire, the weight is gone. The tire's sidewalls relax and expand slightly, causing the internal air pressure to decrease. The air inside the tire has the same amount of molecules, but now occupies a slightly larger volume.
* Temperature: The temperature of the tire can also play a minor role. A hot tire will have slightly higher pressure than a cool tire. The removal from the wheel will usually allow a slightly more effective cooling of the tire.
The change isn't usually dramatic, but it is noticeable with a precise pressure gauge. The amount of change depends on several factors, including the tire's size, the vehicle's weight, and the ambient temperature.
