The devs hid specific combos that break the scoring engine. These are not bugs, but "Easter egg" synergies.
Level 3 of "Interface." It provides diminishing returns in 1.8.6. game dev tycoon 1.8.6 guide
Using the right combinations is essential for "Great" or "Perfect" scores. Popular combos include: Fantasy + RPG on the Play System. Sci-Fi + Action on the Mbox. Sports + Simulation on the PC. The devs hid specific combos that break the scoring engine
The game’s review algorithm is relative, not absolute. Your scores are determined by whether your current game’s and Technology points exceed the average of your previous games. game dev tycoon 1.8.6 guide
Follow this, and you’ll consistently score 9.0+ by year 2005 and unlock the achievement.