Why did Robert Pattinson give current girlfriend FKA twigs a promise ring instead of an engagement ring?

Earlier this week, People reported that the British couple has been rocking their pre-engagement bands for a while.

"He wants to marry her," a mutual friend revealed, describing "The Rover" actor as "much more calm and happy than before."

"They are inseparable when they are together. They live at his house."

According to E! Online, Pattinson actually gave the bling to FKA back in December, sharing the "happy development in his relationship status with a few select friends around that same time at an I-want-to-marry-you dinner."

Moreover, the "Twilight" star was apparently the one who came up with the whole promise ring idea. "It was Rob's idea to get the rings," a source told Us Weekly. "They're 'we're-in-a-committed-relationship' rings."

"FKA loves Rob because he is the nicest guy she's ever been with," the source continued. "Rob sends her flowers, handwritten notes and mp3 files of songs that he's written for her -- she says he's the sweetest guy on the planet."

"It goes both ways," another source claimed. "He says this is the best relationship he has ever been in and he can see it being for life. He admires and respects her."

But why the promise rings? Robert Pattinson originally wanted to get a real engagement ring for FKA twigs over the holidays but "couldn't find the right one, so he held off," an insider told HollywoodLife.com.

Meanwhile, E! Online added that while Pattinson has not shared a timetable with his pals, his proposal could happen as soon as this summer -- after the "Video Girl" singer wraps up her ongoing tour and the "Maps to the Stars" actor finishes up with a couple of projects that he has been working on.