I am working on the Stress Analysis of a Chair, when i work on the optimization i get some strange values, for example my constraint variable is the Safety Factor and it has to be bigger than 2, i reduce the variable that i am working with and it says that it is almost the same SF than before, however if i try to optimize again this time says that it is almost the half, i am talking about the first optimization that i got, so to make it even simpler, i get a value with a X SF, when i am going to optimize that value it says the the X SF is not X anymore but it is around X/2. So my question here is why is this happening? How can i fix this?

Here is the perfect example:

Value of B = 10 Range 5 < B < 20 and any B value has to give me a SF > 2. So far so good but values under 7 gives me SF under 2 but it looks like the software is not respecting the SF > 2 condition because it will give me values under 7 even when that means that i will get SF under 2

Please help me to figure this out