I'm assuming it's the aggregation aspect: the pushgateway will overwrite existing values with the same grouping key. If I'm understanding this correctly, with pushprom each request could send a message like "http.bytes_sent" and you'd get a cumulative total, as you could with the statsd exporter but using the Prometheus data model.
Yes, this is correct. Where pushgateway is specifically not an aggregator we do support it.
Happy to answer any more questions you might have!