Question: A hypothetical scenario:

I've got a project whose source compliance level is specified to 1.5.
Now I compile this project with two different JDKs: At first with JDK 6 Update 7 and then with JDK 6 Update 20.

Do these two different JDKs produce different Java byte code, although they only differ in their Update version?

Answer: 
Let's answer it from the other side: there is no guarantee that any two versions of the jdk produce the identical byte code. So you can expect differences in general.
As usually is for different compilers, it is also in the Java case: the result must be the same but the way it is reached can be (from the bytecodes point of view) different., e.g. because of optimizations or alike. The JVM is stack based V machine; the following is an imaginary example (I don't know jvm mnemonics for the instructions opcodes)

```
push 10
push 20
add
```

```
push 19
push 1
add
push 10
add
```

These produce same result, but generated bytecodes are different (the first is slightly optimized, the second is "totally" not optimized; a third option could be `push 30` since we are adding known (likely at compiletime) constants). This is a simple case, but more complex case can be easily built.