The web server will run under a different user/group combo than your own. As 755 means rwx for user and r-x for group and others (and in most cases the server will fall into the "other"), it makes sense that it cannot write to the folder. If you need it to write there, you must allow write to it.
There's also an option to use ACL (if the underlying filesystem supports it). Something like this:
[dado@liliput tmp]$ mkdir test
[dado@liliput tmp]$ getfacl test
# file: test
# owner: dado
# group: dado
user::rwx
group::rwx
other::r-x
[dado@liliput tmp]$ setfacl -m g:apache:rwx test
[dado@liliput tmp]$ getfacl test
# file: test
# owner: dado
# group: dado
user::rwx
group::rwx
group:apache:rwx
mask::rwx
other::r-x
[dado@liliput tmp]$ ll -d test
drwxrwxr-x+ 2 dado dado 4096 Pro 10 17:48 test
This will give write access to apache group even though it would have it via the "others" permission. Note the + sign after the permissions which denotes that ACL is set.
Also, if your host is running with SELinux enabled (I don't know if this is common for Debian, it is for RHEL clones), you can get permission problems even though it should be allowed (this is called file context)