http://www.bing.com/widget/t/qrcode
Type in a number like 12345 - you will get a QR Code shown on the right.
Now go to a site like https://www.the-qrcode-generator.com/
Type in 12345 in Free Text - the QR Code is different - it's very less dense as compared to the one generated by the Bing widget. I tried all the different options in https://www.the-qrcode-generator.com/ - but none of them give the same QR Code as the Bing widget. And Bing's output scans fine with any QR Code scanner.
This is due to the level of error correction in the barcode. To be precise, the Bing generator can handle up to ~30% of the modules being damaged, where the QR Code Generator link can only handle up to ~7%.
You can tell the error correction level by looking at the two cells closest the bottom left corner, as per the image below:
From here
The more error correction, the more modules required to show the data. Which is fine unless you need to print it small, then the module size is also important.
