TopLink makes no difference between SEQUENCE and IDENTITY strategies (think about it as SEQUENCE-IDENTITY).
That allows the code annotated with these strategies run unchanged on Oracle (which supports Sequences but not Identity) and SyBase (vice versa).
The platforms that support both approaches have to choose one of them.
PostgreSQLPlatform chose to support Identity (indicated by shouldNativeSequenceAcquireValueAfterInsert method returning true).
For identity support PostgreSql apparently uses sequences with pre-defined names - that's exactly what's happening in your case.
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