You probably overestimated your 1RM from the start. If you had an actual tested 1RM you’d probably be more accurate. Heavy lifting is as much physical as it is neurological (CNS). As you get closer to a 1RM your ability to hit multiple reps declines simply because you’re not used to it. I wouldn’t worry so much about your calculated 1RM. Just keep lifting until you can’t hit the required reps, decrease your training max by 10-20 pounds and start over. Of course, if your diet sucks, so will your progress.
You also should be prepared to have bad days. Just because you hit a certain number of reps with a certain weight one week, but on a subsequent week you’re struggling to hit the required reps does not indicate a lack of progress.
Using myself as an example, yesterday on 5/3/1 Squat day (week 6 of a cycle), I hit 395x4 on my 1+ set. That puts my calculated 1RM at 447.6 (445); however, I know my actual 1RM is 475, or even more. In fact, I’m certain I can hit 475 on any “squat” day.
I guess my point is, don’t get all jacked up about the numbers. Just keep at it. And since you’re a novice, keep it simple - think 5/3/1 and the triumvirate, or bodyweight accessories for a while until you start to get the hang of it, and by the hang of it, I mean after a year+ of training. I think BBB for you at this stage is a bit much. You’ll get a lot of progress out of keeping things simple - and your joints will thank you.