You’ll never, and I mean never, do as much single target as the sims say you will. They presume all raid buffs, the best flask, the best food, double potion usage, zero movement, zero mistakes, and extremely low latency (I believe the default presumes something like 15ms). You’re also seeing the average over thousands of sims.
Any individual test (not sim, test) you do on a target dummy is going to be lacking raid buffs (that’s 13.42% damage off the top, due to lacking monk, DH, and warrior buffs), is most likely lacking flask, food, and potion (which is another huge chunk of damage), is subject to the vagaries of RNG and real world latency*, and almost certainly contains at least one mistake over the duration (probably more, even world first raiders make mistakes).
Realistically, if you’re pulling even ~75% of your simmed DPS on a target dummy, you’re doing fantastic.
For actual boss fights, realize that there isn’t a boss fight out there that’s just a straight zerg, short of ye olde Patchwerk. Even the “patchwerk” of this tier, Shad, has a number of mechanics that require movement and attention, and that increases the probability of you making a mistake. Looking at your performance compared to itemlevel, you got 76% on Shad and 97% on Hivemind this Monday. The only one you really did poorly on was Maut, and that’s because you ate a 190k Grand Delusions to the face during Obsidian Skin (really recommend having <40 corruption for that fight, tbh. And a number of others, actually. If you’ve got a spare item, it wouldn’t be a terrible idea to drop one of those Expedient or Severe items for an uncorrupted one until you can fit it in under 40 Corruption in 6 weeks).
The only notable thing wowanalyzer found for Shad is that you cast Cobra Shot while Kill Command was off CD twice, wasting the cooldown reduction effect from it. It also noted that you had Rapid Reload, an exclusively AoE trait, slotted in your shoulders for that fight. Ideally, you should have another shoulder option that has a better single-target trait (like Dance).
*btw, 125ms is actually quite high, when it comes to performance-affecting latency. It’s not Aussie high, but it’s high enough to have a noticeably impact. I typically play at around 30ms, for reference, and I’m just on a normal urban broadband connection (though in the suburbs of Atlanta).
Edit:
Raidbots uses SimulationCraft under the hood, which itself definitely uses Lust.
Do not use AMR. Their theorycrafting is hot garbage, with many of the abilities configured with entirely wrong mechanics (RPPM for percentage proc effects, etc), and they don’t properly “sim”, instead using rules of thumb and approximations to guess your DPS output. Stay away from AMR at all costs, they’re an absolute joke in the WoW theorycrafting community.