Genes are the potential for expression where as actual expression is determined a great deal by environmental influence. This is the same across the biological world. Nature Vs. Nurture anyone?
Lets say we have a strain that has tighter bud growth in cooler conditions and fluffier, loose bud growth respective to warmer temperatures (pretty common). Now we take cuttings from said plant. One cutting is put in an environment where it is exposed to warmer temperatures and one is put in the opposite environment with colder temperatures. There will clearly be a marked difference in growth characteristics even though the genetic material has not changed. Wait now, what if the two are in the "exact" environment you say? Well nothing is ever precise without precision. Meaning, that for most of us actually creating an environment where each plant is getting 100% identical experiences and/or stimulation (environmental pressure) is just not going to happen. I can agree that if you can in fact create an identical environment for each cutting then you will be seeing more uniform growth but again even the slightest variation could result in dramatically different expression. Let's not forget that many genes have already been expressed by the time cuttings are taken. Best wishes