I was checking specification of Canon SX520HS and this is what i found regarding focal length:
Focal Length 4.3 – 180.6 mm (35 mm equivalent: 24 – 1008 mm)
Now to get 35mm equivalent focal length, we generally multiple with factor of 1.6 for Canon but here multiplication factor of 5.5 has been used.
Can someone please explain why this difference in multiply factor or this description is misleading?
A crop factor of 1.6 is used when the sensor is 1.6× smaller (linearly) than a 35mm film frame (usually measured across the diagonal).
In this case, the sensor is much smaller than that — in fact, apparently 5.5× smaller than a 35mm film frame. The SX520HS is a super-zoom compact (sometimes called a "bridge camera", although I think that's a fairly misleading marketing term), and cameras in this category usually have small sensors, which is one of the ways they can have such amazing zoom versatility and "reach" in a package that weighs less than a pound.
You can see in the camera specifications under "Image Capture Device" that this camera uses a 1/2.3-inch CMOS sensor. Read about this "inch" designation here if you're curious, but in practice, this usually means a chip measuring about 6.2mm × 4.6mm. That's about 7.7mm diagonally, and compared to the ~43.3mm diagonal of the 35mm film reference standard, gives a crop factor of ~5.6×. It appears (from the 5.5× value) that this camera's sensor is slightly bigger than that, but not enough to label it something outside of the 1/2.3"-class.]
So, the specs aren't misleading at all. In fact, they show exactly why we still use this reference even though few people use 35mm film (and "full-frame" digital sensors are still high-end gear rather than common in general consumer devices). Using the equivalence makes it easy for consumers to understand the zoom and field of view they'll get from a camera like this when compared to competing models which may have slightly bigger or smaller sensors.
Because the SX520HS is not an APS-C DSLR with a 22.4x15.0mm sensor, which would exhibit the 1.6 crop factor over a "standard" 36.0x24.0mm "full frame" sensor. Its sensor is a 1/2.3", or 6.17x4.55mm sensor, which gives a crop factor of around 5.64.
Canon APS-C cameras with sensor dimensions of approximately 22.5x15 mm use a crop factor of 1.6X. Your SX520HS is not an APS-C camera. It has a sensor with dimensions of about 6.2x4.6 mm.