Why we often observe quite different circadian rhythms, for example, for two plants growing in absolutely identical conditions?

This is might because of the circadian oscillator that affects the circadian rhythm.  The biological oscillations like the diurnal rhythms. It depends on sunrise, the dawn, and the molecular mechanisms by which organisms functioned in this fourth dimension.
I encouraged you to read this paper" https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1425852/"
 McClung CR. Plant circadian rhythms. Plant Cell. 2006 Apr;18(4):792-803. doi: 10.1105/tpc.106.040980. PMID: 16595397; PMCID: PMC1425852.