Waste water abatement - shown on bill?

[Scam]

My other half was reading up on this; https://www.thameswater.co.uk/help/...erstand-your-bill/reduce-your-wastewater-bill

We assume that only 90% of your clean water is returned to our sewers. This 10% difference is called a Wastewater Abatement and is reflected in our wastewater charges.

However on our bill the waste water volume is exactly the same as the freshwater (incoming). The per unit price is different, but it's not 10% different.

Does anyone understand how this works? She's tried the online chat with Thames Water but they're literally answering every other question known to man apart from hers

 

[ttaskmaster]

Surface water drainage from guttering or soakaways, perhaps?
That's not clean water but still an additional contributory factor that may be included on your bill.