i agree with you, but the TRX power is not only a matter of what “should” be done. More often, it is driven by cost.
If you want a high power TRX in DCS1800, it exists, but it’s more expensive. Power supply is bigger, etc.
A TRX1800 at the same price than a TRX900 will always have a bit less output power. I know that 1dB is not neglictible, but i doubt you that anyone has deliberately chosen to have 1dB less in DC1800 ! It’s just engineering constraint for the same cost.