Why? Because the bus companies are private companies trying to make money. Although regulated they often serve what could be various bus lines under the name of one company.
Check the buses front window to find out with line it is within the bus companys number. You could end up in entirely different places if you just rely on the number for direction
Most buses have various ramales (branch lines) that follow various routes (recorridos). For example, the 39, which I would imagines most people on this forum are familiar with, runs from Chacarita to Barracas and has 3 different routes.
The #s 1 & 2 both run along Santa Fe from Ravignani to the center, where they head down towards Consitución/Barracas, but these two lines have slightly different routes, with the #2 (Legally "B") heading up Jorge Newbery to Cramer to Ravignani, while the #1 (Legally "A") follows Charlone/Castillo staight to Ravignani and heads north to Santa Fe.
The #3 runs along Gorriti through Palermo before heading north and meeting up with Santa Fe by Alto Palermo.
Probably the most infamous bus for its many, many routes is the 60, with literally a 1000.
Your best bet to avoid this situation is to mention your destination to the bus driver ainstead of just telling him how much you owe. Although you will not be able to lie about how much you owe, you are certain that you are on the right line.
So, in the future, instead of saying "uno de uno veinte" you should say "hasta Santa Fe y Borges". They will let you know if you are one the wrong line.
Hopefully that little snafu didn't cause you any real problems.
