Stereo VS mono

As you can see in my recommendation, there is a different value for stereo and for mono.
Here is why:

A stereo file at -16 LUFS is composed of two channels, each at -19 LUFS.
Two sources at -19 LUFS add-up to -16 LUFS (+3 dB or +3 LU).

A mono file is usually played back simultaneousely on the left and on the right channel, without level correction.
So, at the end of the game, we have -19 LUFS on each channel, like with stereo, resulting to -16 LUFS!
The only difference is that only one channel is provided... at -19 LUFS.

The following graphic may help you visualize it: