From what I understand, that buffering range is refering to the wide range of products, not their ph range for the Topsoil itself. Indoor mixes are usually buffered to an approximately neutral ph, (7.0ish) outdoor products (compost, mulch, topsoils) are buffered to a lower ph.

If your ingoing water is 7.0 ph, and your runoff really is 6.0 ph, then the Topsoil is likely buffered to 6.0ish ph. Flushing won't improve this, so I'm curious what you're using to determine your runoff ph? A color-coded test kit assumes you are using clear water. Any tinting of the water will skew the results.

You might want to look for an indoor mix that's properly buffered. Or, you can get some garden lime, limestone dust, or dolomitic limestone to buffer the Topsoil, and transplant asap. Practice on an empty pot of the topsoil, as different buffers need to be applied at different rates, and must be watered-in to prevent root burn. Topsoil is usually heavy in manure, which temnds to burn cannabis roots at those quantities. You need a ph pen for this process, though.

Did you check to see if you're rootbound like WW mentioned?