"Trombones, you're late!"
These words have been spoken by many a conductor. Here's another:
"Trombones, sustain those notes, not so separated."
So, there you are sitting back in the section thinking, "Is this guy crazy? I'm starting right with him and sustaining everything!"
Time for our old friend, note shape, to make an appearance.
Look at this drawing:
"Inaudible" refers to any sound you make that can't be heard out there. Meaning, when you factor in the sounds of the other instruments and the distance from you to the conductor, or out into the auditorium, your sound isn't being heard.
If you start a note with a little "wah" there will be a split second when you are playing but they can't hear you out there.
So, to you, the note starts on time. To them, it's late.
You're both right, at least technically.
However, they are more right because, ultimately, the only thing that matters is what it sounds like out there.
This also explains why your lovely sostenuto doesn't sound so sustained out there.
So, look at that drawing. Notice the two horizontal lines. The top line is what you think you are playing (and may, in fact, be playing roughly one foot in front of the bell). The lower line is what they are hearing out there....a less sustained note that starts late.
In the end, out there is the one place that really counts.
Ironic, really, that you'll never actually hear your own sound out there.