Chroma-Subsampling: How to properly calculate the data-rate
Here is a reference to know the difference. I am adding the essential diagram as well: 

  

what you realize is that 4:1:1 has full vertical resolution but 1/4 of Horizontal resolution, where as 4:2:0 has half of vertical as well half horizontal resolution.

However, overall, 4:2:0 and 4:1:1 will have the same number of samples and hence the same bit rate.