The number would be about the leaf, and the question is about the cup
A caffeine figure printed on a box would describe dry leaf. What reaches a cup depends on how much leaf went in, how hot the water was, how long it sat there, whether the leaf was whole or broken, and whether it was the first infusion or the third. Those variables move the result by more than the difference between one tea category and another — which is the finding that undermines almost every comparison a reader has met. So a single figure on a packet would be precise about the wrong thing, and would be read as a statement about the cup. That is a worse outcome than the silence, and it is the reason TeaHQ publishes ranges with a stated basis rather than points.
What the categories can and cannot tell you
The widely repeated ordering — black highest, green in the middle, white lowest — is not reliable and does not survive contact with the variables above. White tea in particular is frequently sold as low in caffeine on the strength of being minimally processed, and a bud-heavy white tea is not low in caffeine at all, because caffeine concentrates in the youngest growth. What IS reliable is presence and absence, and that is a compositional fact rather than a measurement. Anything made from Camellia sinensis contains caffeine. Anything not made from it — chamomile, rooibos, peppermint, hibiscus, linden — contains none. The standing exception is yerba mate, which is a caffeinated holly sold on the herbal shelf, and guayusa and yaupon alongside it.
Decaffeinated is a process, not an absence
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
A decaffeinated tea has been through a removal process and retains a residue. The permitted residual level is defined in regulation rather than being zero, and the wording on the packet describes what was done to the leaf rather than what is left in it. A reader who wants no caffeine at all is better served by a plant that never had any, which is a different question from decaffeination and has a much simpler answer.
What a figure would need in order to be honest
It would need a stated basis: a leaf quantity, a water volume, a temperature, a time, and whether it is the first infusion. Every one of those is a decision the drinker makes after the packet has been thrown away. That is the whole answer, and it is a better answer than a number would be. Where TeaHQ gives caffeine ranges it gives them with that basis attached, and it gives ranges rather than points because a point asserts a precision nobody has.