Mmm let's see..
From what I understand.. ANY strut will deploy Flight spoilers & Right strut will deploy the ground spoilers. And that's a reason you get the occasional smooth landing on crosswinds
In case you didn't forget to arm the speed-brake, that's what you need to deploy them on landing:
-Lever in ARMED & Green light on
-Less than 10 feet RA
-Landing gear compressed
-IDLE thrust levers
-Wheel spin up of more than 60kts
In case you forgot to ARM the speed-brake
, the automatic speed-brake will come up to help you when:
-IDLE thrust levers
-Wheel spin up of more than 60kts
AND
-thrust levers positioned for reverse thrust
For the LDR calculations, Boeing assumes that Reverse Thrust to interlock will be selected 2 seconds after touchdown. Which means that, using the speed-brakes based on the "Forgot to ARM" scenario (& assuming you are fast on that Reverse) it gives you a delay of 2 seconds compared to the normal "Speed-brake armed" scenario.
Now, looking further into those 2 seconds.. Boeing wants an expeditious application of those speed-brakes not only because they are more effective on high speeds; but they also dissipate the lift on the wings and in turn it puts the plane down firmly on their wheels so the brakes can do their job.
Not to forget the Manual Speed-brake correction on dispatch which reduces the field length limited weight and the extra landing distance required on the inflight calculations.
Hope that helps
