This is not what it shows. This image shows the number of addresses that have a balance greater than 1 BTC. This, by no means, should be translated to "people who now own > 1 BTC".
you didn't get the point.
the point wasn't to figure out the exact number of people who own 1 whole bitcoin. the point was the increase itself and this chart shows it very well.
at all times there are people who split their coins among multiple addresses but their numbers don't change that much, when we the slope of this chart increases it shows more people are reaching that target instead of more people splitting their coins among multiple addresses.