confused with swap,free and /proc/pid/smaps show different results

Question

I just ran into a swap problem, so I tried to find which process was using swap, with the script(getswap.sh) shown in this. It was php-fpm, about 200 subprocess, either toke 1M swap space. So I killed php-fpm. Then I ran the script again, and total swap used decreased a lot. Howerver, result in free -m only decreased for about 3M. What is the problem?

before killing php-fpm:

[root@eng /tmp]# bash getswap.sh | sort -n -k5>out
[root@eng /tmp]# cat out|awk '{a+=$5;}END{print a;}'
202076
[root@eng /tmp]# free -m
             total       used       free     shared    buffers     cached
Mem:         64259      60566       3692          0        192      17098
-/+ buffers/cache:      43275      20983
Swap:         4095        155       3940

after killing php-fpm:

[root@eng /tmp]# bash getswap.sh | sort -n -k5>out
[root@eng /tmp]# cat out|awk '{a+=$5;}END{print a;}'
108456
[root@eng /tmp]# free -m
             total       used       free     shared    buffers     cached
Mem:         64259      60402       3857          0        192      17043
-/+ buffers/cache:      43166      21092
Swap:         4095        152       3943

and the script:

#!/bin/bash
function getswap {
SUM=0
OVERALL=0
for DIR in `find /proc/ -maxdepth 1 -type d | egrep "^/proc/[0-9]"` ; do
    PID=`echo $DIR | cut -d / -f 3`
    PROGNAME=`ps -p $PID -o comm --no-headers`
    for SWAP in `grep Swap $DIR/smaps 2>/dev/null| awk '{ print $2 }'`
    do
        let SUM=$SUM+$SWAP
    done
    echo "PID=$PID - Swap used: $SUM - ($PROGNAME )"
    let OVERALL=$OVERALL+$SUM
    SUM=0
done
echo "Overall swap used: $OVERALL"
}
getswap

thx in advance