Heirloom Tomato seeds producing different tomatoes

Last year I bought an heirloom tomato plant and was thrilled with its productivity and fruit! So I kept the seeds, excited for next summer! After successful germination I planted 2 plants from the same seed in pots (same as last summer)

Both plants have produced completely different shaped fruit compared to last year, I am wondering if anyone knows why this is! I’m not complaining, just find it fascinating!

The red and ripe tomatoes are from the first plant I purchased and the picture of the two green tomatoes are from the seed I collected.