I really do not know how a mutation only occur on non-transcribed strand or only on transcribed strand. I think non-transcribed strand and transcribed strand are complementary, so the number of mutations should be the same. Where are my errors?
Mutations occur on one strand initially. To have a permanent change repair or replication needs to change the other strand. So the final mutations will have both strands changed. Repair operates differently depending on the nucleotide changed and on transcribed and nontranscribed strands (presumably because of RNA polymerase etc. coming through on the transcribed strand).
In the example you give, they are inferring the biased repair patterns from observed mutations. They are just not mentioning the complementary change on the nontranscribed strand for brevity and clarity.