14.2/5.8 = 2.4 times increase. This is about 1000% less than the 25x increase. More importantly though, you're looking at the
mean THC % which is an average of all the samples. If you take the most potent sample from the 60s or 70s it will be just as potent as the most potent weed from today, which is the point I've been trying to make. The modest (2.4x instead of say 25x) increase is simply due to the better samples being more frequent due to better growing methods.
If the weed was both genetically more potent than it was in the 60s and there was better access to growing methods you would have a much larger increase than just a factor of 2.4.
psychotomimetic, in 1975 they had Thai samples testing as high as 17%. So obviously the genetics for that sort of potency existed back then. If you have say three 17% samples of Thai stick and eight of 4% crappy outdoor grown weed from the UK bringing the averate down to just over 7% it doesn't mean the weed is stronger in 2010 if there are 11 samples with the average being 14%, right? It just means the stronger samples are more frequent in 2010.
Try and grow out strains that have been preserved like oldtimers haze or any other haze well and have it tested and then decide whether the weed is significantly stronger. I guarantee you compared to the likes of 'OG Kush' it will knock your socks off

If you actually look back at when strains like skunk #1 were actually bred they actually were developed in the 1960s well before the increases shown in the graph. So if the increases in the data don't coincide with the introduction of these strains then it can only be due to one thing.