RRD intentionally aggregates data into larger intervals as the data gets older.

The monitoring graphs are intended to provide troubleshooting information, not be a high-resolution, historical archive. For that you can query the device using something like cacti or zabbix or a plethora of others.

Setting 8 hours x 1 minute resolution is pretty comprehensive. Anything longer than 8 hours and the resolution will be reduced.

You can also achieve this with telegraf and the exec plugin (sysctl)