Comment by epistasis

This is an entirely different issue than code; code mostly does the same thing when you run it twice. There's no such guarantee in biology. A cancer cell line growing in one lab may behave differently than descendants of those cells in a different lab. This may be due to slight differences in the timings between feeding the cells and the experiments, stochastic responses built into the biology, slight variations between batches of input materials for the cells, mutations in the genomes as the cell line grows, or even mistaking one cell line for another.

Reproducibility of software is a truly trivial problem in comparison.