We probably need more information, but the questions to ask - what changed to cause such an increase in the number of stamps? I would think there's a few. The prolific rise and popularity of commemorative stamps. The need of state revenue for newly independent countries or those that are struggling otherwise. Changes in printing technology and lower printing costs. I'm sure there are others.
It looks like they're going off of major Scott numbers, which means the number of new issues is actually grossly underestimated in some cases. For example, in 2012 alone (assuming I counted correctly here), Scott has 108 major numbers for Japan. However, 36 of them were souvenir sheets, mostly of 10 different designs each, and of course it's the sheet that gets the major catalog number, not the individual stamps. The true number of different stamps issued by Japan that year is not 108 but probably closer to 350 or 400. Almost the entirety of Japan's commemorative stamp program is issued as souvenir sheets these days and it's been that way for a number of years. 4126 stamps issued since 1950? If you count a souvenir sheet as 10 stamps instead of 1, I bet they've issued that many in just the last 10 years alone, and they'll probably issue at least 1.5x that many in the next 10 years.
PR China should take into account the stamps issued before Mao's victory in 1949 under the Qing, the Warlords, the KMT Republic and during the Japanese Occupation. Just assigning them to the Rep Of China on Taiwan is not really accurate IMHO. Taiwan used stamps of Japan until 1945, then had its regional issues during the civil war before the establishment of the relocated KMT govt in Taipei in late 1949.
Similarly Guyana should include British Guyana, Australia should include the Australian colonial states and Vietnam should include French Indochina.