'Mech Bays are a subset of Transport Bays that may or may not have Bay Doors:
"BAY DOORS
Bay doors are a veritable necessity for any Support Vehicle, DropShip or other capital spacecraft that intends to carry large cargo items or other units such as BattleMechs, fighters and such within internal bays. As simple in function and design as personnel doors, these larger versions require little more than an equalizing of internal and external pressures before they can be opened to enable the transfer of units or cargo. This simplicity allows bay doors to be mounted as part of the unit’s structure, though doors this large are rarely seen outside of truly large Support Vehicles and DropShips."
- TechManual, p.209
"TRANSPORT BAYS
Even before they became the standard by which all military DropShip amenities are now known, various military and civilian support vehicles—from armored personnel carriers to motor homes—have devoted internal spaces to short- and long-term human habitation or cargo. Today, "bays" is the generic term used to describe a cluster of similar storage spaces for vehicles, personnel or bulk cargo carried by larger vehicles and spacecraft (as opposed to quarters, described earlier in this section). To further differentiate them, transport bays are often clearly defined in the vehicle’s or vessel’s statistics, to express the personnel or equipment they can service and maintain, based on the dominant number of cubicle-style facilities."
- TechManual, p.239
"’MECH/FIGHTER BAY
‘Mech and fighter bays consist of one or more cubicles designed for the long-range transport of ’Mechs, aerospace fighters, conventional fighters or Fixed-Wing Support Vehicles massing up to 100 tons. Because the cubicles are quite heavy and large, ’Mech and fighter bays are much more common to DropShips and truly large support vehicles. Each component cubicle in these bays includes maintenance and/or launch facilities for a single ’Mech (or aerospace craft, as appropriate), as well as basic facilities and bunking for said unit’s crew and technical support personnel."
- TechManual, p.240
From the stats and the images, it isn't unreasonable to assume that the Hysteria has six Transport Bays of type 'Mech Bay, but only four of them have Bay Doors (i.e. launch facilities).