How to interpret the meaning of scan_msg?

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Hi,

This might be a silly question but I have a small doubt regarding the scan_msg in the Laser_scan topic. I am using SICK LMS-200 laser scanner with Pioneer3DX robot for navigation. As I run my code, I notice that the rostopc /scan tells me that scan_time = 0.013s whereas if I check the computation time within my code using something like ros::Time::now().toSec() (present time - past time) I notice that it takes at least 0.1s between each Callback. I am slightly confused as to why there is a difference between the two as the code that I am running is pretty simple and just prints out the elements of the messages in the /scan topic. One guess at it would be that when I run ros::spin(), it goes through all the topics and this takes time. But considering I am not calling any other topic, I am not sure whether this is the reason. Could anyone please help me in understanding what exactly is this scan_msg and why is it different from the Callback time period? Thanks a lot in advance.

Here is the code snippet:

void scanCallback (const sensor_msgs::LaserScan::ConstPtr& scan_msg)
{
   size_t num_ranges = scan_msg->ranges.size(); 
   std::cout<<"Number of range values: "<<num_ranges;
   std::cout<<"\nangle_min: "<<scan_msg->angle_min;
   std::cout<<"\nangle_max: "<<scan_msg->angle_max;
   std::cout<<"\ntime_increment: "<<scan_msg->time_increment;
   std::cout<<"\nscan_time: "<<scan_msg->scan_time;
   std::cout<<"Time difference: "<<ros::Time::now().toSec() - present_time<<"\n";
   std::cout<<"\nNow looking at the ranges**************************** \n";
   present_time = ros::Time::now().toSec();    // for timing calculation
   std::cout<<"\nFinishing one scan";   
 }

 int main(int argc, char** argv)
 {
   ros::init(argc, argv, "laserdata");
   ros::NodeHandle n;
   ros::Subscriber scan_sub = n.subscribe("scan", 100, scanCallback);  // I changed buffer rate over here
   ros::spin();
 }

On adding the header and executing the code, I get the following output:

Number of range values: 181
angle_min: -1.5708
angle_max: 1.5708
time_increment: 3.7037e-05
scan_time: 0.0133333
Header seq: 13070
stamp: 1405978337.781919948
frame_id: laser

Time difference: 0.107676

Finishing one scan

Also, on checking the rostopic hz /scan (with and without running my code), I get the following

average rate: 9.378
    min: 0.106s max: 0.108s std dev: 0.00049s window: 9
average rate: 9.374
    min: 0.106s max: 0.108s std dev: 0.00069s window: 19
average rate: 9.372
    min: 0.106s max: 0.108s std dev: 0.00076s window: 28
average rate: 9.372
    min: 0.106s max: 0.108s std dev: 0.00073s window: 38
average rate: 9.371
    min: 0.106s max: 0.108s std dev: 0.00077s window: 47
average rate: 9.373
    min: 0.106s max: 0.108s std dev: 0.00078s window: 56
average rate: 9.373
    min: 0.106s max: 0.108s std dev: 0.00076s window: 66