Hi Sleepyone,
You can read a little more about Sinomax in post #4 here and while Chinese memory foams can be a little bit more uncertain (see post #6 here) … Sinomax is at least one of the “better” Chinese memory foam manufacturers although the density is always the most important factor in the durability of polyfoam or memory foam and all manufacturers make lower and higher quality foams.
This is not unusual at all with memory foam but at least if it’s made by Sinomax you know that it has been CertiPUR certified.
It varies from manufacturer to manufacturer but it’s fairly common with toppers yes. The “theory” is that having a topper that is an inch less on all 4 sides can help your sheets fit better. The foam layers in a mattress are often smaller than the standard size for a mattress as well because the thicker cover and fire barrier that are around the foam layers of a mattress are part of its final dimensions and these would either be missing or thinner with a topper.
That’s not surprising because you would have an additional 4 1/2" or so of softer materials on top of your mattress which for most people would be more than their ideal and would be quite risky in terms of alignment. The 3" topper by itself on top of your Dunlop topper (which is probably in a medium range) would be a better “match” for most people and would be less risky.
I hope that the topper works out well for you and if it does and you aren’t a fan of memory foam and prefer a more resilient material then it may also be worth considering a softer latex topper as well.
Phoenix