Hello openrouteservice users,
about a month ago we updated our Population database containing the GHSL Data, which produces the population values returned with the isochrones endpoint, due to an erroneous calculation function.
Affected were mostly population values of larger isochrones, while there seemed to be no issue for smaller ones.
For a bit more info please read the following topic
We are sorry for any inconvenience this may have caused. 
Please remember to report any issues like this you encounter, as it is just not humanly possible for us to catch all of them.