If your film was exposed EXACTLY the same and you develop it EXACTLY the same way, you get EXACTLY the same result.
But, a lot of things can happen in a photo shoot. Our shots aren't EXACTLY the same every time. Contrast range of the scene is different every time. So first off, you aren't dealing with identically exposed film, unless you are in a controlled environment and/or you are super careful with your exposure.
Then your developer... Dilution can be off a bit. Temperature may be off a bit. Agitation may be different a little.
The only thing you can really do is to do the best you can to be consistent.
Dilute it carefully and accurately.
Use the same thermometer every time.
Learn to agitate the same way every time.
I use D-76. My stock mixture may be fresh or 6 months old. My dilution may be 5 to 10% off. My agitation may be off by 10 seconds and may not be identical. I do use the same thermometer though.... I get the total development time to say within 10 to 15 second accurate.
This is good enough for me. Any variation, I can compensate at printing time.
I used to use water bath and maintain temp to +/- 0.5C. I used to be extremely accurate on timing including agitation.
It made no appreciable difference for me.
Thermometers can be off by a couple degrees easily, so use the same thermometer every time. Make sure the developer isn't dead. Other than that, I wouldn't worry too much.