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Good Question!
When sequencing RNA, you are usually looking at post-spliced RNA fragments. These reads don't contain any detailed information about intronic nucleotides because by the time they are sequenced, the intronic RNA has been spliced out. The only thing the aligner can tell us is that the read spans an intron. The intronic depth at a location is the number of reads that span the location, but don't have any sequenced RNA for that region.
So, looking at your example, it seems that 150 out of 172 reads simply skip over the exon in question. This probably means that the exon is poorly expressed and gets spliced out most transcripts. However, in the 22 cases where the exon wasn't spliced out, 20 had a mismatch. I'd say you have some good evidence for a variant, but I'm not sure it's conclusive. A common standard for calling a variant is to have at least 30 reads that support it.
