Off topic:Genome and Transcriptome as a reference

Question

[KVC_bioinfo]

Hello,

I have mapped RNA-Seq data with human genome and transcriptome. I observed there is a great difference between the overall mapping percent. The overall mapping percent with transcriptome as a reference is very less than overall mapping percent with genome as a reference.

Could someone help me understand the reason why this is so?

Thank you in advance.