What kind of ph test are you giving? A color-coded test srtip or kit...? Runoff ph will show different, because the tint of the water skews results.

The peat-based soil is always low. That's why the soil companies add lime or somehinfg similar to buffer the mix back to neutral. But as time goes on, the buffer starts to degrade, and lose it's buffering capabilities.
Is this runoff reading from fresh soil, or has it been used a time or two? You might want to look into getting some garden lime, some fresh soil, or changing the brand altogether. A runoff of anything under 6.3 in soil is unacceptable.