Mega Shine has 2 Immersive (3-Star) cards, 1 regular Mew and 1 shiny Mew. Despite these cards being categorized as the same rarity, the shiny Mew is actually rarer than the regular one. You can expect to pull the regular one in around 1 in 90 packs (same probability as Immersives usually are), but you can expect to pull the shiny Mew in around 1 in 300 packs. However, the tracker seems to not account for the shiny being much rarer and treats the 2 variants as if they were 50/50.