Hi, pysam prints me reads that has different chromosome (ref_ID) than the one that I specify with samfile.fetch("chromosome", 100, 20000). The following is my example code:

import pysam
samfile = pysam.AlignmentFile("ex1.bam", "rb")
reads = samfile.fetch("chromosome", 100, 20000)
for item in reads:
    print (str(x))

When I use samfile.fetch("X", 100, 20000) it prints reads that their chromosome number is 10, this is one of the reads for example:

K00114:267:H3YL7BBXX:4:2122:14874:29659 163 10  130 255 14M83903N136M   10  84334   150 GAAAATGAAATAAGCAAAGTTAGGACTCGTAAAAATGGCAATCAAACCAAGAACGAAGGGCAAAACGTACTCCTCAAGATCGGTGGGTTCGCAGTGGTTCAACAGGCTTGGTTTCAAGCAGAACAAGTACGGAACTTGTAAATTTTTGTC

But when I use samfile.fetch("IX", 100, 20000) it prints reads on chromosome 4!, one example:

K00114:267:H3YL7BBXX:4:1124:28980:37712 419 4   17  3   22S12M86722N116M    4   86873   128  TGCAACGTCCACAAGCAGTTCTCACCACACCACATCAACAAGTTCTTACACATCTTCTACTTACACTGCACAAATATCTTCTACCTCCGCTGCTGCTACTTCTTCTGCTCCAGCAGCCCTGCCAGCAGCCCATAAAACTTCATCTCACAA

or samfile.fetch("V", 100, 20000) print reads for chromosome 6! one example:

K00114:267:H3YL7BBXX:4:1201:16346:16805 419 6   137 0   148M2S  6   217 148 CAAACACTAAATCAAAACAGTGAAATACTACTACATCAAAACGCATATTCCCTAGAAAAAAAAATTTCTTACAATATACTATACTACACAATACATAATCACTGACTTTCGTAACAACAATTTCCTTCACTCTCCAACTTCTCTGCTCTA

There is something strange here, does anybody knows what is the possible source of this error?