A pot of dal is usually better on the second day, while a vegetable sabzi rarely is. The difference is about what state each dish was already in when it went into the refrigerator.
Dal has nowhere left to degrade
Cooking a lentil to softness has already broken down its cell structure completely. There is no texture left that storage can ruin, because the dish is a uniform suspension.
Water content is also already uniform, so there is no moisture gradient for the refrigerator to flatten out. Nothing moves from one part of the dish to another.
Reheating simply raises the temperature of a homogeneous mixture, which is why it comes back essentially unchanged apart from thickening as it cools and thinning when stirred with water.
Flavor continues to develop
Aromatic compounds from the tempering and the spices keep diffusing through the liquid while the dish stands, distributing more evenly than they had at the end of cooking.
Starch released from the lentils also continues to hydrate, which gives day-old dal a slightly fuller body. Both effects work in the dish's favor.
Vegetables depend on structure that storage removes
A sabzi is made of pieces that retain some firmness, and firmness depends on cell walls that continue softening while the dish sits warm and then cold.
Moisture also migrates out of the pieces and into the surrounding space, so browned surfaces that were dry become wet and the contrast the dish was built on disappears.
Reheating cannot restore either, because both changes are one-directional. The pieces cannot re-firm and the surfaces cannot re-dry inside a covered container.
Fried and crisp elements are the worst case
Anything crisp is crisp because it is dry, and a sealed container with warm food in it is a humid environment. Crispness is lost in the first hour.
This is why fried components are stored separately when they are stored at all, and added at the point of serving rather than cooked into a dish destined for the refrigerator.
Planning around the difference
A cook batching for the week gets more value from making dal, beans and slow gravies in quantity and cooking vegetables fresh, which is also the faster half of the work.
Cooling and storage practice matters for both, and the published guidance from a national food safety authority on cooling cooked foods and refrigerated storage times is the appropriate reference for how long a batch should be kept.