Quick clarification.

BER is always caused by a calcium deficiency at the growing point.

This can be caused by 3 things: insufficient translocation of calcium from the roots to the growing point, lack of calcium in the soil solution, or calcium unavailable for other reasons.

Insufficient translocation: The most common cause of the disorder. A few different causes are possible: irregular watering, rapid temperature changes, sucking insects, or any damage to the root syatem (fungus, bacteria, insects etc.) It can also be genetically related for example the old OP Roma are notorious for it.

Calcium unavailable in the soil. This usually is related to pH. If the soil pH is above 8.5 or below 6.5 the plant can't take it up. Calcium like all other nutrients can also be temporarily locked up in the microbial community with high amounts of organic matter being broken down.

Lack of calcium in the soil solution. This is pretty rare for most gardens and the least likely culprit. Its the easiest to fix however so this is generally where most people start.

Most likely irregular watering. It is caused by calcium deficiency but the answer isn't to supliment with more most of the time. The calcium in the soil just needs the water to be able to move from the soil to the roots