cannot be true unless all the bison were born on or very near the same day, every year for the 30,000 years represented by the tar pits. If there was any variation (as there is with modern bison) they would've had to move very rapidly through Rancho La Brea staying only a few days at the most...
This site tells me that bison mating season starts in July and can run through September. That's a spread of three months. They give birth therefore in Spring/early summer.
We shall say then that birthing starts in April, running through to June. One month later (July) mating season starts again, and one month after that they arrive at the tar pits (in August). This means that the calves that were born in April (the eldest) will be 4 months old; the youngest calves, born in June will be only two months old.
In this case, you are right - if they stayed a few months, we should expect to see 5 and 6 month olds.
However, if birth happened closer to the time of their arrival things might be different. Especially if younger calves are treated differently. I can't find any particular information that would be definitive either way. However, at two months old, the calf begins to develop its horns and humps. I'm also reading that most calves
are weaned by late summer. The latest calves to be weaned take seven months, but it seems a few get their earlier.
Perhaps the calves we see in the pits were weaned? We'd need more information to be sure (the nature of the teeth wearing for example).
Alternatively, as Lithoid has said, it could be that we are getting the information worded within error bars (that is, the ages of the calves found are 3 months plus or minus one month).
That said, kudos for have a critical mind and percieving the strangeness of the numbers. We need to look at some primary literature concerning the aging of these bison, so we can establish how the museum have interpreted it for easy consumption.