I agree that must be annoying! Could be a number of reasons though. A heavy approaching Kilo has called ready, or is anticipated calling ready, and you are sent there to reduce WT. Or, a heavy is about to taxi and with more than 2 acft at Juliet, he would have to wait until they go before he can continue on to Kilo, so JST, VOZ and TGG all go Sierra/Kilo and QFA to Juliet. We don't always anticipate correctly when the heavy will call ready so that may be why you get stuffed about sometimes. Also the ADC might spot a gap that can be used by a Medium at Juliet, but isn't sufficient to get the heavy away, so again you miss out. It might be that the one at Juliet is an early left turner (turbo prop going left 260 or a jet on a CRENA/ESDIG) - again we can usually get two away with reduced separation upwind as opposed to two both on a DOSEL (this scenario is 34 only, but works similarly for 34 deps to NE with an off mode 34 dep). In your situation above, you might be given Juliet after all if you tell ground you'd be ready at Juliet - they may be able to offer it to you. If you've already switched to tower and call ready approaching Juliet, we have to check with ground to make sure you can have it, otherwise we might upset their sequence, and they may be busy so there would be a delay in us getting back to you.