Interpreting read coverage over gene body plot

Hi,

I’m working on some RNA-seq data for my thesis and I was hoping that someone could help me out.

My sequencing library was prepared using Illumina TruSeq Stranded mRNA kit and sequenced with a NovaSeq sequencer. After read alignment I did some post-aligment QC with RseQC and now I have this read coverage over gene body plot, which I am not sure how to interpret.

The curve starts at 0 in the 5′ end of the gene body, but clips before it reaches 0 at the 3′ end of the gene body.

Why is that? Does that mean that there is some sort of a bias? I looked up similar plots on the internet and the ones that have a bias are generally more slope-shaped.

Thanks!