Why is there a big difference between the actual mined rewards and the theoretical ones?

The theoretical reward or the result of the reward calculator only serves as a reference for miners.

The theoretical reward is the result of a calculation that is based on network rewards and difficulty—it is a temporary value. If the volatility of a network’s difficulty is large or the speed of a network’s difficulty changes quickly (such as ZEC, ZEN), the theoretical reward of that coin tends to change more frequently. Therefore, there will be a significant difference if momentary values are used to estimate the mining rewards for an entire day. 

The payout scheme is also an important factor to be taken into consideration. For example, BTC applies the FPPS scheme ( the basic PPS rewards and transaction fees are included). The theoretical reward and the actual mined reward differ since the transaction fees are unpredictable. The miner rewards shown by the mining reward calculator are based on the data from the past 8 hours. There will be significant differences if the total transaction fees from the network change a lot. 

In the PPLNS payout scheme (DOGE, KAS, CFX, etc.), the mining pool calculates rewards based on the actual blocks mined. Rewards are affected by the pool’s luck value, which can cause daily mining rewards to fluctuate significantly.

Payout schemes: PPS, PPLNS, FPPS, PPS+

Luck value

Blocks mined by f2pool

Blocks Mined and Luck Value records

Therefore, the mining reward by the calculator can only be considered an estimation.