This adapter was specifically designed to provide 5.25V not 5V, but we still call it a 5V USB adapter. We did this on purpose to solve a problem that occurs often with USB-powered gadgets: they draw so much current than the resistance of the cable causes a voltage drop, so instead of 5V, the device sees 4.75V or so. To avoid this problem, we made the adapter 5.25V. This is because the USB power specification allows up to 5.25V, and its safe for all 5V electronics, and has the benefit of making up for any loss due to thin USB cables. Basically, you can use it where ever USB 5V power is needed, with no risk of damage, and it will happily work at the full current range, no matter what cable you use!
This adapter ( https://www.adafruit.com/products/501 ) was supposed to give me the right voltage for my PI. It's rated at 5.25V/1A. However, I'm only getting a 4.4V-4.6V on TP1/TP2, even if I disconnect mouse and keyboard. As we know that's below the 4.75V recommended minimum.
I replaced it with a larger 5V PS and I'm now getting 4.92V.
I still like Adafruit website as it has a nice selection of accessories for your PI, but this power supply should be avoided.