An octal mode encodes bits, not a complete access decision. The same 0755 converts to rwxr-xr-x for either a regular file or directory, but x means program execution for a regular file and search or traversal for a directory. Ownership, process credentials, ACLs, mount options, and other policy still matter.
Read means content versus names
For a regular file, read permits reading its data when the rest of the access decision allows it. For a directory, read permits listing entry names, but it does not by itself grant access to metadata or contents behind those names. Directory search permission is a separate x bit.
Directory write controls entries, not file data
Write on a regular file permits modifying its data subject to the environment. Write on a directory participates in creating, removing, and renaming entries and normally needs search permission to perform useful pathname operations. Deleting a directory entry is therefore not the same question as writing the referenced file.
A converted mode is not a command guarantee
GNU chmod can preserve setuid or setgid on directories when a three- or four-digit numeric mode does not explicitly address them; five-or-more-digit, operator, or explicit symbolic forms can behave differently. Symlink dereferencing, recursive traversal, filesystem policy, privileges, ACLs, and ownership add further context. ChmodCalc models the requested 12-bit snapshot and never promises the observed state after a command.