Penalty Metrics - NFL - DPI vs. DHLD

[hefnerjm]

So, the other day when I saw what appeared to be an obvious DPI called as a DHLD, and realized that over the last couple of years, I feel that Defensive Holding seems to be getting called more, and DPI much less (at least at the pro level).  So I was curious to do some research. 

Below is a list of average penalties by type per team over the last 6 years.  Please note that 2015 is current through 10 weeks, not a full season, hence the lower average # of fouls per team.
Year - DPI - DHLD
2009 - 5.94 - 3.41
2010 - 6.47 - 3.59
2011 - 6.56 - 4.03
2012 - 7.91 - 4.78
2013 - 7.72 - 5.66
2014 - 6.72 - 7.34

2015 - 4.41 - 4.56

All metrics gathered from http://www.nflpenalties.com/?year=2015

I notice a significant line of demarcation from 2013 to 2014.  DPI is down significantly year-over-year (7.72 to 6.72), and DHLD is up significantly (5.66 to 7.34). 
My hypothesis is that this is due to the severe penalty for DPI, and how it can seriously impact the outcome of a game, whereas DHLD has a less significant immediate impact. 

Anyone have some insider knowledge whether the NFL Officiating dept is placing emphasis on officials to "when in doubt" make it DHLD instead of DPI?

[riffraft]

No insider knowledge, but a point of emphasis came out before the 2014 season on Defense Holding. It was emphasis enough that the browns corners and safeties were wearing a boxing type glove during training camp that year to help them defend without the grab

[bossman72]

No insider knowledge, but a point of emphasis came out before the 2014 season on Defense Holding. It was emphasis enough that the browns corners and safeties were wearing a boxing type glove during training camp that year to help them defend without the grab

Agree.  For instance, now they want ANY jersey stretch called as DH or DPI.