"A cross came in and a defender and the goalkeeper [Jonty Underhill] left it for each other so, yeah, it was poor defence from us."
The Danny Wilson-led Rovers assumed control and had ascendancy into the second spell but centremid Justin Gulley found the net in about the 55th minute to make it 2-0.
"We were still in the game. Oli Chapman hit the crossbar and Josh Stevenson had a shot which was cleared off the [goalmouth] line."
A staccato of counter-attacks yielded three goals in the space of 15 minutes with Mason-Smith claiming two, Gulley just as many and William McIntyre one.
Rightback Phil Dol scored a consolation goal for the Blues in the dying minutes.
The Rovers comfortably beat a weak Upper Hutt 4-0 in their opening round at Park Island, Napier, but Miramar was going to be a litmus test.
"It was obviously a good test today and we competed well.
"We had our chances but, unfortunately, we didn't take them and they did."
Robertson said had his troops scored a goal from Chapman or Stevenson, the complex of the game would have been totally different.
"By no means is our season over. There's a long way to go," he said as they prepare to travel this time to Fraser Park, Wellington, on Good Friday for a 2.30pm kick off in round three.
Rangers coach Valerio Raccuglia said Beckett's sending off was fair as he was the last man.
"That's the rule and we accept that," Raccuglia said.
The Italian said Miramar adjusted well and didn't hit the panic button.
"We changed our midfield, putting four in there so we had a 4-4-1 formation and we kept the ball."
Raccuglia said he agreed with Robertson's after-match speech that had Rovers scored, the game might have had a different ending.
The Rangers were without No1 goalkeeper Phil Imray (injured) while ex-Rovers player Cole Peverley and Cory Chettleburg were serving one-match suspensions.
In other matches on Saturday, Petone held Wellington Olympic to a 2-2 draw, Wairarapa United pipped Palmerston North Marist 2-1, Lower clinched the Hutt derby 1-0 and Western Suburbs thrashed Tawa 4-1.