Thanks for the checking my sketch cattledog, I did some more tests like you suggested and here are my observations. Also I found out the model of my receiver module, it's called "LR45B".
If you switch receivers between the nano and the uno, does the problem follow the receiver?
I tried switching the Arduino boards and Yes the problem follows the receiver module. Maybe the library is not meant for this type of module?
I also tried another superheterodyne module called RXB12 and the results were the same as LR45B. When my Generic 433MHz Rx module reads 51, LR45B and RXB12 reads 40. So both superheterodyne modules are reading the same value.
Update: Oops my bad, The reading was correct for the superheterodyne module when connected to the first arduino nano. what could possibly be the problem??
Are there any other transmitters in the area?
Nope, I have only a single 433MHz transmitter, nothing else. No other sources of 433MHz transmission in my knowledge.
How are the three arduino's and their antennas arranged?
The Generic 433MHz modules have a simple 15cm single core telephone wire as their antenna in vertical position. LR45B modules has a spring coil copper antenna that came with it and has about 21 turns and is 2-3cm tall. RXB12 has no antenna attached. They all receive the signal at a distance of about 8M through one reinforced concrete floor.
Do things change if they are moved to a different arrangement?
The generic module has difficulty receiving signal if its antenna position is tweaked and stops receiving signal and also has somewhat poor range. LR45B module receive signal whether it's coil antenna is placed horizontally or vertically and has better reception range than the generic module. RXB12 receives the signal without any antenna when placed horizontally.
Are both receiver arduinos printing at the same time, once per second?
Yes they are. The arduino built in led indicator blinks at the same time when they receive their signal and also prints at the same time.