The number in grey (Total Available Beds) it represents the number of beds/room available in your property for that room type. Meanwhile the number in white represents the number of beds/room bookable online (Online Availability). Every time a booking is made through Hostelworld Group, the number of beds/room in grey will reduce, as well as on-line availability, in order to avoid overbookings.
If the number of total availble beds is higher than the availability, it means the availability has been manually reduced.
Here is an example: 6 bed mixed dorm for March 31st, there are 3 beds available in the system as the other 3 has been already booked through Hostelworld. However, only 2 beds are bookable online, the other bed is on hold by the property.