Initially, when Google released it's highly anticipated Pixel devices, the product page for the products stated the same peak charging of 18W. At the time, this didn't seem impossible due to the fact that the Pixel and Pixel XL are near identical on the inside, the only differences being the display size and resolution and the battery size.
Interestingly, though, an avid Android tester decided to put Google's claims to the test by recording data from the smaller Google Pixel device and, as it turned out, the Google Pixel's charging maxes out at a 15W peak, somewhat lower than the 18W claimed by the company. Once this data was released by the tester, Google confirmed that there was an oversight in the product description and has confirmed that the Google Pixel can charge with 15W while the Pixel XL can charge at the peak of 18W which was initially claimed for both devices.
For those now wondering whether the company's other battery-related claims are also wrong, there's no need to worry. This oversight should not affect daily use of the Google Pixel in any way and the charger that is included in the box will still work, the only difference is that it limits charging at a lower wattage than originally advertised, though the claims of 7 hours of battery life with just a 15 minute charge should still be valid. The reason for the limit in peak charging is unknown, though it likely is to do with the fact that a higher wattage could cause overheating due to the smaller size of the battery, something the company is sure to want to avoid after the chaos the Galaxy Note7 has caused. Also, by limiting the charging peak, the company can claim the same charging numbers for both devices due to the fact that, while the Pixel XL charges 20% quicker than the regular Pixel, the larger sibling has a battery that is roughly 20% larger, so the difference in charging speed shouldn't be noticeable.
For many consumers, the charging wattage is not something they look into, but for many enthusiasts, it is an important spec. It remains to be seen if the company will receive any complaints after admitting to its mistake, but with reviews of the device being so positive, it's difficult to see why this would happen, especially considering it shouldn't affect daily use of the device in any way.