Phineas is not the most observant boy on the planet. Nor is he the the most considerate.
He, like everyone else cuts Ferb off when he speaks. Ferb doesn't quite understand why people do this. Sure, he doesn't talk a lot, but it's not like he's silent.
Of course, most of the time, Phineas does understand what's going on in Ferb's head. They're on the same wavelength, usually. And often enough, Ferb is happy to let Phineas do the talking for him. Phineas, after all, loves to talk. And Ferb… well… doesn't. He prefers to think before he speaks, to make certain it's the right thing to say. And usually, by the time he's decided, someone else has said it for him.
Yes, Ferb communicates much better silently.
Phineas, on the other hand, is much more… not outgoing, because nobody can say that Ferb is shy, but more open. Ferb keeps things to himself. Phineas couldn't keep a secret to save his life- or the life of anyone he cares about, either.
Still, Phineas has the best heart of anyone Ferb has ever seen, so he's willing to overlook his brother's flaws (the oversharing and tiny bit of overbearing are coupled with the fact that Phineas is just a bit ADHD and lets Ferb make the plans most of the time- it's not that Phineas can't, it's that Ferb is more methodical), because he knows Phineas loves them all. When he interrupts Ferb, it's because he genuinely hasn't realized his brother wants to speak. The speed at which he changes topics and jumps from idea to idea are because he's just too excited to stick to one thing. And, honestly, Ferb wouldn't want him to be a liar. Phineas is open. It's who he is.
They're a perfect match, and Ferb wouldn't have it any other way.