Axial analysis: What does it mean when most attributes of some lines appear "-1" value?

[ctrl-WJY]

I did a axial analysis of a road net of a city to calculate every road line's depth and integration[HH]. When checking result, I found some lines' integration values are "-1". The same value also appears in Entropy, Mean Depth, Intensity, Node Count and some other attributes of these lines. I really wonder what these "-1" values mean. Does it mean these lines are irrelevant, not significant, or disconnected? The situation is displayed in the attached picture (Blue ones are those lines with values of "-1"). Appreciating any help from you.

Wen,
From Sun Yat-sen University, Guangzhou, China