That first round has one fench clinch from Rousey and one reversal in a clinch which ends up with her attempting an armbar on Holms back. Again as I said watch the fight and I think its pretty clear Rousey is not focused mostly on grappling, she's trying to KO Holm, loading up on punches and not looking to push in for a takedown behind them.
Look at the start of the second round with Harrison and you see the obvious difference, she backs Holm up with a series of strikes then is looking right for the takedown. Earlier in Rousey's career she looked to do that more often, latter on it was mostly looking for strikes, she would take the opportunity for a takedown if it was offered but she wasnt nearly as focused on it anymore.