IP::stats age problem

I am using an iRules pretty much the same as the following to calculate the response time of a HTTP request and respond. 
when HTTP_REQUEST {
    set reqAge [IP::stats age]
    set reqURI [HTTP::uri]
    set reqClient [IP::remote_addr]:[TCP::remote_port]
 }
 when HTTP_RESPONSE {
    set respTime [expr {[IP::stats age] - $req_age}]
    log local0. "Client at $reqClient requested $reqURI. \
Server response was received $respTime milliseconds after the request was sent to the server."
 }

But when I looked into the log. There are negative number in the Response Time field(TCP_AC = CLIENT_ACCEPT, HTTP_RQ = HTTP_REQUEST, HTTP_RP = HTTP_RESPONSE, RT = Response Time)

TCP_AC,2010-09-20 18:28:54,12849785341334,8,00000, Age:130

.

.

.

HTTP_RQ,2010-09-20 18:29:00,12849785341334,8,00017, GET,www.nownews.com,{| http://www.nownews.com/common/ticker.htm |},{| mozilla/5.0 (windows; u; windows nt 5.1; en-us) applewebkit/534.3 (khtml, like gecko) chrome/6.0.472.59 safari/534.3 |}, LR:-52, Age:5882

HTTP_RP,2010-09-20 18:29:00,12849785341334,8,00018, ST:200, CT:text/html, CL:1973, RT:-23, Age:5859

And I saw some connection age were reset when response:

HTTP_RP,2010-09-20 18:20:46,12849780461284,8,00002, ST:200, CT:text/plain, CL:10, RT:-119, Age:2

HTTP_RP,2010-09-20 18:20:46,12849780461285,8,00002, ST:200, CT:text/plain, CL:10, RT:-138, Age:2

HTTP_RP,2010-09-20 18:20:46,12849780461286,8,00002, ST:200, CT:text/plain, CL:10, RT:-138, Age:2

HTTP_RP,2010-09-20 18:20:47,12849780471287,8,00002, ST:200, CT:text/plain, CL:10, RT:-157, Age:2

HTTP_RP,2010-09-20 18:20:48,12849780481290,8,00002, ST:200, CT:text/plain, CL:10, RT:-159, Age:2

Could anyone explain this? Many thanks.