Look at diaries of the same genetics from the same breeder. If you are comparing different strains, its apples to oranges. Different growers also have different concepts of when flowering begins. With some it's when the sex first shows. With others, it's when pistils appear in abundance. For me, it's when the stretch begins. You could consider that preflower, but it's definitely through the veg phase. You could have a 2-3 week difference on these points alone, so try not to worry too much. Make sure you have switched to bloom nutes which are more P/K and less N. This will encourage bud development.