问CDN回源的几点问题？

按下CTRL+f5响应如下：

Response Headers

view source

Access-Control-Max-Age:1800

Cache-Control:public, max-age=7200

Content-Length:146125

Content-Type:text/html; charset=utf-8

Date:Fri, 10 Nov 2017 05:40:50 GMT

Expires:Fri, 10 Nov 2017 07:40:50 GMT

Last-Modified:Fri, 10 Nov 2017 05:40:00 GMT

Proxy-Connection:Keep-Alive

Server:nginx

Vary:Accept-Encoding

via:proxy A

X-Cache-Lookup:Hit From Upstream

X-Cache-Lookup:Hit From Upstream

X-Cache-Lookup:Hit From Upstream

X-Cache-Lookup:Hit From Inner Cluste

X-Daa-Tunnel:hop_count=3

X-NWS-LOG-UUID:87b69fe3-8907-47ca-a826-e91a274b9b8e 13fd2f902e54c7a44871a863d6fb2931

按下f5响应如下：

Access-Control-Max-Age:1800

Cache-Control:max-age=7200

Content-Length:0

Content-Type:text/html; charset=utf-8

Date:Fri, 10 Nov 2017 05:41:49 GMT

Expires:Fri, 10 Nov 2017 07:41:49 GMT

Proxy-Connection:Keep-Alive

Server:NWS_SP

X-NWS-LOG-UUID:94967c37-fa51-43ac-8291-04381c6383cd 13fd2f902e54c7a44871a863d6fb2931

再按下ctrl+f5响应如下：

Access-Control-Max-Age:1800

Cache-Control:max-age=7200

Content-Type:text/html; charset=utf-8

Date:Fri, 10 Nov 2017 05:48:17 GMT

Expires:Fri, 10 Nov 2017 07:48:17 GMT

Last-Modified:Fri, 10 Nov 2017 05:40:00 GMT

Proxy-Connection:Keep-Alive

Server:NWS_SP

Transfer-Encoding:chunked

via:proxy A

X-Cache-Lookup:Hit From MemCache Gz

X-NWS-LOG-UUID:5e807fd6-fbb9-474d-9751-c178a768516f cb241fe1eaa83309dc13288eec0e1cb5

再按下ctrl+f5响应如下：（问题1：明明在内存中缓存了为啥又在磁盘中命中？）

Access-Control-Max-Age:1800

Cache-Control:max-age=7200

Content-Type:text/html;charset=utf-8

Date:Fri, 10 Nov 2017 05:51:35 GMT

Expires:Fri, 10 Nov 2017 07:51:35 GMT

Last-Modified:Fri, 10 Nov 2017 05:00:00 GMT

Proxy-Connection:Keep-Alive

Server:NWS_SP

Transfer-Encoding:chunked

via:proxy A

X-Cache-Lookup:Hit From Disktank Gz

X-NWS-LOG-UUID:62de3fa1-15c1-472d-b579-e4b4cc8dc407 0c2871257850ac0aa3ce73273410bb98

再按下ctrl+f5响应如下：（问题:2：明明上次请求在磁盘中命中了，为啥又回源了？CDN配置的缓存时长也没有失效的。）

Access-Control-Max-Age:1800

Cache-Control:public, max-age=7200

Content-Length:146125

Content-Type:text/html;charset=utf-8

Date:Fri, 10 Nov 2017 05:53:37 GMT

Expires:Fri, 10 Nov 2017 07:53:37 GMT

Last-Modified:Fri, 10 Nov 2017 05:20:00 GMT

Proxy-Connection:Keep-Alive

Server:nws_ocmid_hy

via:proxy A

X-Cache-Lookup:Hit From Upstream

X-Cache-Lookup:Hit From Inner Cluste

X-Cache-Lookup:Hit From Disktank3

X-Daa-Tunnel:hop_count=2

X-NWS-LOG-UUID:8060f4d8-7e51-4035-bfe9-ef2ce7f8c1db a9a19a62e1b08ac7e93bf230d0b98dca

如上问题1、2。CDN团队能帮忙解答下吗？