Actually, that is the only way it makes sense: The normal 7.62X51 tracers (orange color code on bullet tip) are designed to start trace at some distance from the gun, and sometimes a metal cup in the rear of the case provides this delay, as it must be "burned through" to ignite the trace mixture. In the described training "overhead fire" the guns are mounted fairly close to the "crawl space" the trainees are traversing, and a "delay tracer" would not have the time to ignite, and would provide the troops no input.
By using an "immediate ignition" tracer such as the predominant tracer types from WWII, the trace would burn as it passed over the trainees, providing some stimulus. Immediate ignition tracers have tradionally been identified by the red bullet tip color.
The worst option would be to back up the firing point from close proximity to the training or "crawl" lanes: This would allow a chance for greater bullet, dispersion, greater bullet drop, less accurate fire, and less intense input to the trainees from the noise, flash, etc. resulting from the firing. So, we would have less training gain with greater risk to the troops.