The base attack for the bite of the giant squid is 2D6+7 indicating it is a primary attack. The 'tentacles' attack is 4D6+3 indicating it is a secondary attack. However the constriction damage for the giant squid is 4D6+10, how did the developers come up with that?
There are no hard rules on how to define constrict damage
And this irritates a lot of community members. Each developer simply decides what would be a reasonable damage for a monster's constrict ability when designing them. As such, we have a wild variety of constrict damage values, as can be seen on this answer.
The description on Constrict says that the damage is typically equal to the damage caused by the creature's attack:
The amount of damage is given in the creature's entry and is typically equal to the amount of damage caused by the creature's melee attack.
But some creatures will treat the constrict as a primary natural attack and cause strength bonus times 1.5 (like the assassin vine), others will cause strength bonus (like the corpse lotus), others will treat as secondary and cause half strength bonus (like the giant octopus). Some creatures we have no explanation for the strenth bonus on their constrict damage, and we can't tell if they are primary, normal or secondary (like the gray ooze). Others will cause a damage that is completely different from the attack that originated the constrict damage (like the Yaoguai).
For the Giant Squid (and a few others), while the tentacles attack is considered a secondary attack (50% str bonus), the constrict ability is treated as a primary attack and gain 150% str bonus instead of only 50%.