Shell Programming and Scripting

I am using bash shell from quite a long.But i could not understand how the shell is working for the below example.The shell i am using is bash.

a=5
a=2 echo $a (no semicolon after assignment)
shell output is 5.

a=4 cat <<EOF
$a
EOF

the output will be 4.
Any idea ? For the first case i think shell is executing the last command only if no separator is there.Simple example like
a=3 a=6 a=7
echo $a
the output will be 7
But what about the second cat ??????

truss it or tusc it