Bitrate of the video changes after exporting from Media Encoder with hardware encoding enabled

I want to export my Premiere Pro sequence via Adobe Media Encoder in 20Mbps, constant bitrate (CBR). In the 'Bitrate Settings' tab, I chose the hardware encoding method. When the video is rendered, and I check the file's properties, the bitrate shown is much lower and so is the file size. 

However, when I use the Software Encoding, the bitrate perfectly matches to what was written in Adobe Premiere Pro. 

What could be the problem?

My CPU is Intel Core i7 9700 KF

My GPU is Nvidia GeForce GTX 1650