Well what you wanted would be a two-level grouping feature: parallel at the bottom level, and sequential at the top level. That is more complicated than what the firmware can do at the moment. The sequential groups are suitable in cases that different zones are on different water supply lines therefore each group has to run in sequence but different groups can run in parallel.
In your case, you can always set all zones to the parallel group and then just use multiple programs to schedule different zones to run at different times.