Let's say I type in the address bar: "the best way to". Here are the suggestions:
And here are Google's suggestions:
If you go to IE's advanced settings, you'll notice that you can enable or disable "get suggestions from Bing as I type". To get Google's suggestions, you can go to google.com, install Google's app for Windows Phone or install a different browser.
So why would Microsoft show Bing suggestions if you've changed your search provider to Google? Maybe it's an excuse for Microsoft to send all your queries and partial queries to Bing, so that it can improve its search engine.
Update. Hadrien Zero-Un has an explanation in the comments: "Internet Explorer for Windows Phone can't have Google as a search engine. BUT Nokia modified the browser to allow the change of the search [engine]."