Difference in random values generation in zsh and bash subshells

I am not able to understand why zsh is generating the same random value when calling a subshell.

Consider the following code:

$ cat script.sh
#!/bin/zsh

checkFieldConvergence () {
  echo $RANDOM
  echo $RANDOM
}

echo $(checkFieldConvergence)
echo $(checkFieldConvergence)
echo $(checkFieldConvergence)
checkFieldConvergence
checkFieldConvergence
checkFieldConvergence

$ ./script.sh
4049 24768
4049 24768
4049 24768
4049
24768
20764
3330
17114
1195

whereas the same with bash gives

$ cat script.sh
#!/bin/bash

checkFieldConvergence () {
  echo $RANDOM
  echo $RANDOM
}

echo $(checkFieldConvergence)
echo $(checkFieldConvergence)
echo $(checkFieldConvergence)
checkFieldConvergence
checkFieldConvergence
checkFieldConvergence

$ ./script.sh
12274 28155
27609 10269
14100 14662
6945
17897
20354
29817
14495
27552