LB its like looking at a weather forecast. Pre-flight if the forecast indicates that you have to carry fuel to hold for 60 minutes or carry an alternate then the regs say carry it. Once you get airborne the weather changes so now you dont have to carry the fuel but it is on board anyway. Another example is ETOPS. Before take-off there are a whole bunch of rules that govern how much fuel you take and what level of equipment must be serviceable. When you are airborne basically you do what you have to do to complete the flight safely. So with the pre-flight LDR calcs you will work out if you can take the payload based on the forecast weather. Those calcs have different assumptions built into them. Once you get airborne (there is that phrase again) you work out if you can actually land at your destination based on the actual conditions and actual landing weight with different assumptions built into the calcs. So if you worked out pre-flight that you were limited then you might have reduced your fuel (tankering possibly) or payload. Inflight you are no longer interested in what might have been you are only interested in what is actually possible with your actual weight.
Hope this clears up your confusion.