Sometimes they're just actually behind too! I'm a professional organist, and recently made my continuo debut with a pretty high level orchestra. I wanted to make a good impression, so at the rehearsal, my eyes were glued to the conductor, he gave a clear downbeat, I came in with him...and was at least half a second ahead of everyone else. I was eventually able to figure out how to adjust, but those first few entrances were rough.
After the rehearsal, I mentioned it to my boss, and he confirmed it's his experience with orchestras as well. They're somehow able to come in together but definitely behind what the conductor shows, while organists are used to staying exactly with the beat pattern. He said it's particularly annoying for things like the Saint-Saëns Organ Symphony, where the organ's supposed to come crashing in on a huge chord directly with the orchestra.