Per cent means “per hundred”. As such, 100 % is “it all”. When talking about growth, more than a 100 % makes sense, meaning that the total is bigger than before, thus the comparison might include a value higher than 100. So far, so good.
However, and this has been going on for quite a while, there is a tendency to say a value, higher than 100, although not talking about growth. Often, it’s to describe certainty or performance. I believe it was started in the sports world, where a player might say, “We need to go out, and give 110 %!”. Giving more than you have is, by definition, not possible. Sure, there are concepts such as “over perform”, but apparently it is within this player’s reach to perform this. Thus, not more than a 100 %. You can’t be 110 % sure, because 100 % is sure, you can’t be more than that.