The phone's fast, 10 watt charging only works with a Made for Google-certified charger, like the Pixel Stand.
Your brand-new Pixel 3 or Pixel 3 XL offers wireless 10w fast charging, but only with Google's own Pixel Stand or with upcoming Made for Google-certified models thanks to a proprietary protocol. Qi-standard chargers, even if they support 10w, are limited to 5w for Google's devices.
The fast-charging incompatibility was pointed out to Android Police by a reader, and the site also says that the phone reports fast charging with third-party devices even though they're only operating at 5w.
Google confirmed to us,
We don't limit third-party devices, in fact, we're working with our partners in the Made for Google program to get fast 10W chargers certified for use with Pixel 3 (Belkin announced its 10W Pixel 3 charger already which will launch in the coming weeks). Pixel Stand and Pixel 3 work together through the protocol we've developed for fast charging. Everything else charges at the industry standard Qi 5W.
Google hasn't indicated why it took this approach, but it's possible it's linked to the extra features that are activated when you pop the Pixel 3 on a Pixel Stand.