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At this point in time the TR's were still had a Solid Axle in the rear with the rear axle mounted with leaf springs. On the front section between the towers note the support between the towers to negate the flexing inwards of the spring tower assembly.

Now once we move to the TR4A the rear of the frame changes somewhat. This model introduced Independent Rear Suspension (IRS). The rear section is moved inwards. This enables the mounting of the rear control arms for the suspension with the addition of frame sections and mounts for the differential unit.

To fit the suspension in the rear of the frame the mounting for the control arms or “trailing arms” are set at an angle. I imagine to allow room for a single arm giving a good amount of travel and also plenty of room for the spring. Also using what amounts to a raised cross-member to carry both the differential and locate the springs. The downside to this arrangement is the pivot points of the trailing arms are offset making the hub or wheel scribe an arc as it travels though the suspension movement. So basically, the wheel is only at the correct position at one time in its travels.
It could be argued that as Body roll occurs (as there is no rear anti-roll bar) the suspension allows the corresponding outside wheel to move upwards into the wheel-arch and the tyre will have a flatter profile to the road. Like-wise as the inside tyre moves downwards relative to the body the same will happen as they move through their arc. I must say I have never tried jacking up one side of the car to see and note the result.