Pressure washing concrete


New member
Just started pressure washing as a side gig but would like for it to one day become a full time opportunity. I pressure washed this concrete driveway today. Once I finished the job I noticed these random dark spots that were on the concrete. The driveway was completely dry. The concrete is 20 years old. Was this an error on my part or just the aging from the concrete over the years?? Thanks in advance. Here are some after pics