Transcription Factors

Due to alternative splicing. Within the genome, there are enhancer regions where enhancer-binding proteins, called transcription factors, attach to and allow the DNA to be copied. In other words, these transcription factors "enhance" the replication process of certain genes by binding to these enhancer DNA sequences. Sometimes functionally related genes (2 or 3 of them) are transcribed all at once, and then are later spliced so that only one genes leaves the nucleus and gets translated. All three genes, for instance, are controlled by one enhancer region.

This is my guess, I'm not too sure if it's on point though.

There are only a small number of different transcription factors and DNA control elements. They act in different combinations, and can establish highly specific and precisely controlled patterns of gene expression in different cell types.