|Ed Girallon Poe|
The Spriggan does not follow the Monster Advancement Size Chart. If it did, it would lose 4 Dex upon going Small -> Large.
Giant Crab, Goblin Dog, Llama, Giant Leech, Monitor Lizard, Panda, Stag & Thylacine are all animal (or vermin) companions that goes Small -> Medium and does not follow the Monster Advancement Size Chart.
There are plenty of animals that goes Medium -> Large and gain different stats then the Monster Advancement Size Chart claims.
Granted, Animal companions could probably have just been worded as 'follow the normal rules for monster advancement', but instead each and every one was given their own listed stat bonus. Just like the Spiggan : )
There is one part of the monster advancement section you are forgetting. "These values are not absolute." When the developers and editors look over the final product for those creatures, they skewed them for their intended role. That does not mean that the other numbers do not fall within the charts capabilities.
The spriggan and animal companions stat modifications for size follow monster advancement, with the stated variation, and most certainly do not follow the polymorph size chart.