"Lactate Threshold" in cycling terms is actually a combination of true "LT" and VO2-max HR. Real LT is simply a muscular force. When combined in cycling, gearing comes into play. Repeat your LT tests at 90rpm, 95rpms, 100rpms, 105rpms at the same speed and you'll find that the "LT-HR" will vary based upon gearing.

Typically, LT-HR will be higher when using lower gears and spinning at 100rpms. Power-output at LT will tend to be a little higher as well, depending upon your neuro-muscular coordination at that RPM. At lower-RPMs, LT-HR will be lower along with lower power-output due to the higher forces needed on the pedal.

The trick is to balance your muscles vs. your heart. If your legs hurt but lungs/heart feel fine, then use lower gear and spin more. If your lungs/heart hurt, but legs feel fine, then use a bigger gear and push harder.