In the GDP calculations, the Official Inflation will "deflate" the GDP number calculated in today's money (the Nominal GDP) to produce the Official GDP number (the Real GDP), so if the Official Inflation is lower than reality, the difference in inflation pretty much "leaks" into a higher GDP number than it should be, or in other words, officially it looks like GDP growth.

You might have noticed just how much politicians celebrate that they've "made the Economy grow" using the GDP Growth figure.

Massaging the Inflation is an easy way to create GDP "Growth" in the official figures and is far more indirect and obscure than massaging the GDP figures themselves so less likely for those doing it to be caught and even if they do only a handful of people actually understand the true significance of what has been done.