Hi Ben,
This comes down to a decision that was made in Junos a long time ago (when the EXs were first released).
Basically there was an argument about whether VLAN membership should be attached to physical ports, the way it was done on the M, T and MX routers at the time, or whether physical ports should be assigned to VLANs, the way other vendor's switches did it at the time .
In the end, both methods were implemented, which is why the GUI and CLI lets you do things both ways.
If you go into Switching in the GUI and add a port to the VLAN, your config change will be:
[edit interfaces]
+ fe-0/0/5 {
+ unit 0 {
+ family ethernet-switching;
+ }
+ }
[edit vlans VID11]
+ interface {
+ fe-0/0/5.0;
+ }
if you go into the physical port and add the VLAN there, it will be:
[edit interfaces]
+ fe-0/0/5 {
+ unit 0 {
+ family ethernet-switching {
+ port-mode access;
+ vlan {
+ members VID11;
+ }
+ }
+ }
+ }
My personal preference is assigning VLAN IDs to a physical interface (the second one), as it is a lot easier to see (from the CLI anyway) what is native and what is tagged on a port all in one place.
But in terms of functionality (and your original question), both configuration options achieve the same functionality, even if the CLI they generate may be different.
Hope this helps!