Last-Modified 与 If-Modified-Since 相同，但返回200，而不是304响应？

Question

1、我在 openresty 的 nginx.conf 中设置了：

location /mem_modi {

default_type text/html;
set $memc_key $arg_key;
memc_pass 127.0.0.1:11211;
memc_flags_to_last_modified on;                  # 设置 Last-Modified 标记

}

反复请求 /mem_modi?key=foo，发现Last-Modified的值与If-Modified-Since的值相同，但一直返回的是 200 OK 的 HTTP 响应码，而不是 304 (Not Modified) ？这是什么原因？

以下是 HTTP 的报文信息：
Request URL:http://*:9999/mem_modi?key=koo
Request Method:GET
Status Code:200 OK
Remote Address:127.0.0.1:19180
Referrer Policy:no-referrer-when-downgrade

Response Header:
Connection:keep-alive
Content-Length:5
Content-Type:text/html
Date:Fri, 29 Mar 2019 02:22:44 GMT
Last-Modified:Fri, 29 Mar 2019 01:54:16 GMT
Server:openresty/1.13.6.2

Request Headers
view source
Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,/;q=0.8
Accept-Encoding:gzip, deflate
Accept-Language:zh-CN,zh;q=0.9
Cache-Control:max-age=0
Host:*:9999
If-Modified-Since:Fri, 29 Mar 2019 01:54:16 GMT
Proxy-Connection:keep-alive
Upgrade-Insecure-Requests:1
User-Agent:Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.75 Safari/537.36