The ATC Flight Plan is (usually) generated by the Dispatch Section who are aware of approximate aircraft weights and capabilities, and also the winds en route, therefore it has best climb points. After take-off when Atlantic entry point ETA is known, or closer to Gander Oceanic entry point, accurate ETAs can be sent and an Oceanic Clearance at a single level is issued. No enroute climbs are involved at that stage. It is then up to the crew during the Oceanic phase to request reclearance at any time to whatever level they deem appropriate, which does not necessarily correspond to whatever level was in the preflight clearance. Proximity to another aircraft just above you can keep you down. Sometimes it is worth climbing earlier than optimum to get out of cloud tops.