I think it’s quite easy to overlook that Los_Angeles and GMT-7 aren’t the same thing.
LA is in either PDT (GMT-7) or PST (GMT-8) depending on the time of year, and the clock changes occur in different countries at different times (UK: last Sunday in March and last Sunday in October, vs US second Sunday in March and first Sunday in November) so the offset will vary.
That’s why date/time systems usually show both the time zone and the offset.