LT bike vs run - why is one higher?

Are you making the mistake of using your run LT test numbers for your bike??

You need to do separate tests for each discipline if you want to accurately train using HR.  Except in very rare instances, people will always have decent variations in the LT levels for each sport.  In fact, that goes for any different sport.

Typical....

Running takes more of the body (upper body works a lot too while running), so running LT HR will be higher than for biking.  Swimming is even lower than both of those since it primarily upper body and does not use the major muscles of the legs as much as the bike and run.

I have found my running LT HR is about 7-8 bpm higher than biking...

Yes, that is the rule of thumb, but needs to be tested in individual field test like Daremo said.

I'm not trained specifically in human structure/motion, so this is a layman's interpretation of these fields. 

Think of the various muscles used in running/cycling:
Calf-absorb/rebound the footstrike/limited flexure during pedal rotation
Thigh-absorb impact, move lower leg/move lower leg
Hip-move upper leg, stabilize torso/move upper leg (some stabilization)
Torso-coordinate arm/foot motion, stabilization/limited stabilization
Shoulders-swing arms to counterbalance leg motion/limited stabilization
Neck-stabilize head/hold head in proper position

Running engages a larger group of muscles and with higher intensity.  The result is your running LT is usually 8-10 bpm higher than your cycling LT.  (And your swimming LT is 8-10 bpm lower than cycling. )