I'll also add that Klingons go about conquering others. I'm sure they came across some planets and civilizations with advanced engine designs or computers or deflector technology or transporters, and whatnot.
Klingons conquered these planets and pillaged the technology, perhaps making greater advances than they could on their own.
Also, depending on what tech they came across and stole, they could have varied and inconsistent history. Maybe a huge leap in deflector systems after they stole some tech from a conquered world, but no matching advance in weapons for years. Or engine tech stagnates for decades, but weapons improve greatly. Not all Klingon tech progresses equally from within, it would vary as they went out conquering and taking tech from their subject worlds.