part of it is that they want to maximise the # of people seated at any given time...full tables mean more profit, of course. So there are less odd-number tables listed in the bookings break-down within the computer system...it's not really a glitch so much as a booking strategy built into restaurant systems. (although a party of 3 would be seated at a table meant for 4, given the choice between seating a party of 3 at that table, or seating a party of 4 at the same table...they would rather sell 4 dinners than 3 dinners!)
So even though there is a set number of actual 4-top-tables which CAN accomodate either 3 or 4 people, they may only designate as few as 10% of the tables in the whole place for parties of 3. Once those 10% are booked, a party of 3 would be told no more tables, when in fact there are lots of tables for 4 left open.
Go ahead and take the table for 4...you will not be charged for the "no show" if 3 of you show up (they only charge the "no show" if no one in your party shows up.)
The same is true for parties of 1 (take the table for 2), and parties of 5 (take the table for 6.)