Remember when TMZ Sports spotted Jimmy Butler drivin' around Beverly Hills in a freakin' minivan??

The Philadelphia Sixers superstar is finally dishing out the deets on why he prefers mom mobiles over luxury whips ... saying it's all a big scheme to throw people off.

And, that it does.

Not only does Jimmy Buckets rock vans -- he was in a Toyota Sienna when we shot him back in September -- he also has a "baby on board" sign in the back to top it all off.

"I'm telling you, you would never know it's me swanging that bad baby," Butler says.

"I stay real nonchalant with everything."

Hey, like we said ... the Sienna ain't a bad car at all --  0 to 60 in 7 seconds with "spacious, high-quality interior," according to U.S. News and World Report ... and those 14 cup holders!!

It's all part of Butler's new YouTube channel, which documents his life on and off the court.

The rest of the vid is pretty interesting ... documenting his first day as a Sixer ... which includes a funny clip of getting lost in the team facility.