There are two things couldof happened.
First in the numberingdialog box in the “Special” tab it is an option “Reuse unused marks”, thisoption will use the unused marks for the new elements you have detailed.
Second themodifications you did can group or/and ungroup elements in the model, the new elementscreated will take the pieces marks of the identical beams already existing inthe model.
Let imagine you haveyour model and you have beams from B1-B10 quantity 3 for all of them. Let’s saythat after some modifications B4 and B7 are empty piece mark the 6 beams (3xB4and 3xB7) are now group with other beams. After other modifications one of thethree B2 is not anymore identical with the other two B2 so when you do thenumbering if the option “Reuse unused marks” is checked the piece mark will B4because is the next available. Same thing will happen if from B9 we remove one,after the numbering it will get the B7.
If the option “Reuse unused marks” is not checked in our example the new piecemars will B11 and B12.
So actually you can see that both cases are working together.
The fact that you havethe same sp mark is not an issue; you can have different piece mark as anassembly with the same sp. If you weld or bolt in shop any objects will notchange the sp drawing but the main part yes, so using the same sp you can havemany assemblies.
I hope this willanswer to your questions about the numbering if you have other question don’t hesitateto ask us.