Weight limit on top tether anchors?

I am currently taking a CPS course and today we were talking about the weight limits on the LATCH system.

I knew of the Lower Anchor weight limits; some are 40, some 48. And when you dont know what it is for your car you divert to the lowest limit.

But what I didnt know of, was that there are limits on the top tether anchors as well. My instructors said that all vehicles have a limit of 40lbs on the top tether anchor. And they looked it up in the LATCH manual.

But what I dont understand is that, why do all the higher harnessed seats reccomend using the top tether when it is only useable to 40lbs? This just doesnt make sence to me.

Then there are some that say you MUST use the top tether after a certain weight. I think I have read (here) that after 60 lbs you MUST top tether the Regent.

The instructors said that you should not tell parents to use the top tether past the limit and that if the seat manual says it MUST be used then that seat is incompattable with the vehicle and shouldnt be used. Uhhh, WHY WOULD CARSEAT MANUFACTURES HAVE THESE GUIDELINES IF NO THEY CANNOT BE FOLLOWED IN ANY VEHICLE???

It just doesnt make sence.

Can someone please explain this to me?

Someone else asked this yesterday (lots of y'all in class this week!) Tell your instructors to refer to the LATCH manual, here's the commentary on top tethering to higher weights: http://www.saferidenews.com/html/LATCH_P49.htm

Julie gave the best answer/advice. Read the link.
Caregivers have to make the tough decisions. There is much confusion on the top tether issue on these high weight harness seats. It is likely that the seat belt, whether LS or LO, will survive the stress of the HWH seat during a crash,depending on the severity, although the belt system may need replacing afterward.
Top tethers have been shown to reduce head excursion.
Additional questions that need to be answered are as follows:
1.Will the top tether fail with a HWH? This is always a maybe as every crash is different. The tether could fail with a regular CR in certain crashes.
2.If it does, will it have sufficiently reduced the excursion and aided the HWH in maintaining its integrity?
3.Will non-use of the top tether on a HWH reileve the restraint manufacturer of liability if the restraint fails?

If the resting weight, child and restraint, exceeds the vehicle manufacturer's recommendation for the top tether, the vehicle manufacturer's case is resolved to a certain point (speed and direction of impact are similar to sled test in the "dynamic pulse" test used in vehicle LATCH anchor testing).

If a caregiver can resolve question 2 & 3, then the answer on wheteher or not to use the top tether could become less confusing.