There is no single Indian spice mixture, and blends carrying the same name differ substantially between regions and between households. The variation is functional, and it follows from what each blend has to work against.
A blend is calibrated to the rest of the dish
Spice mixtures are assembled to sit against a specific cooking fat, a specific souring agent and a specific staple. Change any of those and the balance shifts.
A blend built for coconut oil and tamarind is designed against sweetness and sharp sourness at once, while one built for ghee and yogurt is working against richness and a milder tang.
Using either in the other context produces a dish that tastes crowded in some directions and empty in others, which is why the blends did not converge.
Roasting changes the ingredient before it is ground
Many blends dry-roast their spices before grinding, which drives off moisture and triggers browning reactions in the spice itself. The aroma shifts from raw and bright to deep and nutty.
Different regions roast to different degrees, and some blends deliberately leave certain components unroasted so their volatility survives into the mixture.
Because roasting is judged by smell and colour rather than by time, this is also where household variation enters most strongly.
Where the blend goes in the dish
Some blends are cooked in fat at the start, where their compounds are extracted and dispersed. Those tend to be built from robust spices that tolerate heat.
Others are sprinkled at the end and never really cooked, so they are made from the most volatile aromatics and would be wasted in a long simmer.
A cook who reverses those two roles gets a weak result in one direction and a raw, dusty one in the other, regardless of the quality of the blend.
Wet blends solve a different problem
Coastal and southern cooking uses wet ground pastes as much as dry powders, combining fresh coconut, chilli, ginger and herbs with the dry spices.
Grinding wet keeps the aromatics dissolved in the paste rather than exposed to air, so the mixture holds its character but must be used within days.
Dry blends trade that freshness for storage life, which is why they dominate in regions where the wet ingredients were not available year round.
Why household versions persist
Blends were traditionally made at home in quantities meant to last weeks, using proportions passed within a family rather than written down.
Commercial blends are formulated for broad acceptability and long shelf life, which pushes them towards the middle of the range and away from any regional extreme.
Cooks who find packaged mixtures flat are usually noticing exactly that flattening, and the common remedy is to add one or two freshly ground spices on top rather than to abandon the packet.