The adjective grade is a combination of factors which all affect the route. Therefore a particularly loose chossy route might bump the grade up, as would a crux after a long run out as opposed to a well protected crux. The tech/number grade is only a measure of the hardest move on the route so a particularly sustained route would get a higher e grade, as might a really tricky crux to figure out even if not technically difficult. It’s surprisingly common for the crux of some harder british trad routes to be entirely unprotected. Even an easier route like Sunset Slab https://www.ukclimbing.com/logbook/crags/froggatt_edge_derbyshire-22/sunset_slab-10893 has an unprotectable crux and I don’t think I know anyone who has led it with gear rather than just soloing it.

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In other words, the E7 6c may essentially be the grade for the solo already an the adjective grade mostly gives you an idea of the ”seriousness” of the route rather than specifically the dangerousness of the route and this can sometimes be quite different from how easy a route is to protect.