It's not the real situation, because they are including bonus cushion. By the deadline we will have, if nothing goes wrong, ~$4.4M (not including Savard on LTIR).
Sure it's a real situation. 4.4 million may be the total cap space they'll have available however guys like Iginla will be owed only approx 1/3 their current contract at the deadline as their team will have paid out 2/3rds already. 17 mill is the total of yearly salaries Boston is projected to be able to acquire at the deadline, not actual money. Boston can add Iginla because his cap hit at the deadline will not be 7million, rather it will be 2.something million. That said...
You don't include bonuses until they're hit. You plan for them as a lump sum total at the end of the season but you don't include the cushion in cap equations before hand else you skew the final totals. The max lump sum total Boston will have to plan for is Dougies number ($212,500) and Seguins (2.65mill) which equates to $2,862,500 and chances are all those bonuses won't be hit. So it will affect that final total, but not in a dramatic fashion. Come deadline the FO will have a decent idea of what bonuses will likely be hit and which ones they don't have to plan for. That 17 million total will drop because of it, but not likely that drastically.
Another factor that will likely drop that total, and will affect it more immediately will be call ups. If they bring up Caron or Krug for a few games their daily total will apply to the Bruins cap and lower that final number somewhat, though still not likely that drastically.
Boston won't be able to acquire 17 mill worth of contracts come deadline day, but 10-12 mill is very realistic and a relatively safe bet (assuming no acquisitions between then and now of course).
There's no reason to think Boston will be limited by cap space at all this year. They'll only be affected in regards to acquiring cap hits that extend beyond this year.