I haven't inspected the insides of the particular relays involved here, so I will just make a few comments about mechanical relays in general.
The difference in opening & closing force between the electromagnet & the spring has already been covered & correctly described. That is one piece of the puzzle.
Another piece is the wiping effect on the contact surfaces. The opening & closing of the contacts causes a slight bending of the contact arms, or a compression
of the arm retainer springs. This causes one contact to slide sideways across the other, wiping away the byproducts of arcing. The wiping geometry is different when opening compared to closing. Siemens made great inroads with the robotic welding equipment
in UAW plants in the 1970's with improvements in this aspect of relay design.
Although it is not always labeled on the outside of the relay case, the contacts also have different current ratings for different types of current & load conditions. AC ratings are different from DC ratings. Inductive loads have different ratings compared to resistive loads. The worst possible combination for most mechanical relays is high voltage DC on an inductive load.
There can also be duty cycle ratings, especially with definite purpose relays.