The simulation models don't take other extraneous variables into account like:
- Metallurgy (Build composition of the airframe)
- ECM and radar jamming capabilities.
- Altitude (you know if the jet is hugging tree line almost in sub-sonic and can lower it's heat signatures can drastically be less detectable).
- Turbulence and air drag (Can be simulated with CFD solvers but still real time situations is difficult)
These simulation model only interpret what radar reflection can come from every angle of the plane, i.e. geometrical radar cross section verification. But these extraneous variables can often affect the RCS on any jet. You can take few Hawk trainers, modify it a bit to carry one or two bombs internally and fly it just few hundred feet's above tree line ... there you go, your own stealth jet, the problem is to maintain your course with respect to the terrain which not only requires a sharp ass avionics system but also a top gun level'd pilot.
Certainly new air combat systems have these variables into play, when they go flying. So stealth works.
Detecting a single F22 or two of them is much difficult then entire squadron in an contested airspace, which I am mostly sure that the rules of flying a stealth jet our bit more different.
That's why F22s that are being detected in Syria might be very far away, and very few in numbers. We can learn a lot from Red Flag exercise. The engagements with a single or two F22 were seen from a long distance until a 4th gen pilot got lit with MWR (Missile warning receiver) all over it's cockpit.
If we put our thinking cap for bit more, it's certainly realisable that it's not that radar's can't detect them, the problem is, F22 were in the air space with very confidential AOA i.e. They were either approaching the 4th gen jet with a certain AoA that paint's it's RCS very low (Frontal aspect certainly), or they let the 4th gen jet come closer to them, without burning too much of there fuel and remaining sub-sonic (reducing IR signature). Thus a mind game happens here. Either the 4th gen jet pilot verify it by breaking the formation or just go with the usual course. If it breaks the formation to go Recon mode, it will verify for F22 pilot that certainly there is situation over here with multiple bogies, now an F22 pilot with AN-APG71 can put multiple radar locks from very far away and shoot BVR AAMRAM and leave the place (increasing distance so that AoA get's more blurry). By the time 4th gen jet reach a bit of historical distance it's too late, a homing missile is on lock on mode.
When I look at B2, It's also still stealthy, but you have a huge plane, yes at certain AoA it might appear as small jet. However, I am bit sceptical about B2 stealth characteristics. In current radar tech. S400 was designed to detect it and can certainly shoot it down when thrown in few numbers.
Coming over to J20, it's detectable with new radar tech, maybe the extraneous factors I formerly discussed, some of them yield no value to it's RCS. That's why you can differentiate between True stealth from fake ass stealth.