A reader has been researching tea for a little while now and has found that steeped, loose tea has less caffeine in it than comparably sized cups of bagged tea. They want to know if this is true and why that difference would exist in the first place.
Well, the issue of caffeine levels in tea is slightly more complex than a teabag vs. loose leaf tea issue. Amongst other things, it has to do with the "grade" of the tea leaf (how broken it is or isn't).
A lower grade of tea is more broken, so it has more surface area. The higher ratio of surface area to leaf volume changes the rate at which various compounds infuse and increases the overall rate at which the tea infuses. As a result, tea ends up releasing more of its caffeine when it is broken than when it is whole leaf (and it releases it more quickly).
Not all teabags are broken leaf and not all loose tea is whole leaf, so this doesn't ALWAYS mean that teabags have more caffeine than their loose-leaf equivalents, but it usually means that.
One other factor is the tea varietal. Many Assam teas and African teas (both of which are commonly used in teabags) are grown from the Assamica varietal, which is higher in caffeine than the Sinensis varietal.
I hope that clarifies things a bit! You might also find my article on factors influencing caffeine levels in tea to be helpful.