Do young children grow in spurts
Growth is neither even nor constant, It can vary quite a bit from month to month as hormone levels change, the growth curves you are seeing are a rough average. 

You are having a hard time finding anything because it one of those pieces of knowledge that is so old people only rarely research it directly focusing more on deviations from it and you will likely only find good reference to it in textbooks. For a layman this means searches based on the terms you think you should use never find what you want, something as simple as adding the word "textbook" can make a huge difference. Google scholar will also help. 

http://adc.bmj.com/content/51/3/170.short

https://books.google.com/books?id=YA-LdtyHJ9QC&amp;pg=PA226&amp;dq=evolution+of+the+growth+spurt&amp;hl=en&amp;sa=X&amp;ved=0ahUKEwjkl4zCztjRAhVB74MKHRiuB7kQ6AEIHzAA#v=onepage&amp;q&amp;f=false