Hi Tubalubs,
There would be a reason but if you are comparing apples to apples in terms of the same type of latex and components then it would be more about their business model and the other factors that can be involved in how any business prices their products than anything to do with the mattress itself (see here). They also sell through dealers rather than selling factory direct.
If you are comparing the same type and blend of latex then they would be the same quality/durability yes although FSF (and Savvy Rest) uses organic Dunlop and SleepEZ uses 100% natural Dunlop. There is more about the different types and blends of latex in post #6 here.
When you can’t test a mattress in person … the best source of guidance for comfort choices is a more detailed phone conversation with the manufacturer themselves who will know more about their own mattresses than anyone else and can help “talk you through” the options they have available that have the best chance of “matching” different body types, sleeping styles, and preferences based on the “averages” of their customers that are similar to you (see mattress firmness/comfort levels in post #2 here). For most people this will be “close enough” that any further fine tuning that may be needed can be done by rearranging layers or with a layer exchange.
You are certainly looking at some great options and any one of them would be a qreat quality/value purchase and when you are down to final choices that are between “good and good” and there are no clear winners between them then your final choice (see post #2 here) will really be a matter of “best judgement” based on your conversations with each of them and all the objective, subjective, and intangible parts of your personal value equation that are most important to you.
Phoenix