Hitable means something different as you increase the base difficulty. On the higher difficulties, you can see 20 or more pitches sometimes without getting one in the middle of the zone, so what's hitable in those circumstances is anything that catches any part of the zone. The only way you can make the CPU throw a lot of pitches up the middle is by setting the difficulty to something below All Star. On the difficulty levels I've played in many different versions of RTTS, I've had several stretches during every season where the CPU mostly stays on the edges of the zone and tests your patience. RTTS has always been like that.