Raj Kumar ji has given great insight by sharing how HR in manufacturing firms have slightly different role than the one in IT industry.
Mr. Govind sums it up well - the ability to pay.
I would like to just point out the differences further -
In manufacturing firms, IR is more important than HR (as is explained by Raj Kumar ji)
You have labors working, there would be unions, there would be added efforts to communicate and negotiate with them, change management would be very different etc.
Though when we do MBA we are given an introduction to the IR as a small subject as a part of the curriculum but compared to the vast information given in HR, IR is I felt just touched upon.
More so, IR is more into knowing the laws and how to deal with blue collared people than having the gyaan of HR. So the experience matters more here.
Secondly as rightly said by Mr. Govind, IT companies are way ahead than manufacturing companies when it comes to profit generation and hence distribution.
Again I would like to mention that the said difference is not limited to the HR department only, if you would compare the payscale of say the CEO of the company, there would be difference in more or less same or similar ratio.
Usually the pay-scale differs from industry to industry because of several factors.
Also note that when we are paying there is a specific ratio to be kept between the designations that are prevailed in your firm. I mean we cannot have the HR head and the Marketing head have huge difference in their pay scale. They might not get the same and obviously there would be varioations but not very huge and very obvious.
Hope it helped.