I would think the problem is that egg-rolls and spring rolls are clearly two different things but the numbers of differences can very.

On the main egg-roll article, there is a citation that links to an article written by no less of an authority than Chinese-American chef Martin Yan.  There Yan details the main difference between the two.  Egg-rolls have eggs as part of the wrapper, along with wheat flour and water.  Spring rolls just have the wheat flour and water. 

This one ingredient different causes several changes.  The inclusion of the eggs causes egg-rolls to be darker, slightly chewer and with a rougher outer texture.  Spring rolls, with the lack of egg, fry up slightly crispier with a lighter color and a smoother outer texture.  All of this is directly from Chef Yan, not myself, so it can be taken as authoritative.

And all that is clear enough.  The problem or confusion if you will, most often centers around the filling and the size.  At this point, we mostly have personal experience.  Your average Chinese restaurant in America is bound to have a dish called an egg-roll.  This egg roll is going to have the wrapper made from egg, wheat flour and water.  It will be filled with assorted vegetables and a protein.  The protein can be shrimp, pork, chicken or a combination of shrimp and pork.  The differences in the protein are often regional, with some areas only having shrimp and/or pork while other areas only having chicken.  The size can also be different as well depending on how thick or thin the cook rolls the egg-roll out.  I've seen egg-rolls thicker than manicotti or as thin as spring rolls.  The only thing uniform is the color and texture as the effects of the egg in the wrapper are consistent.  In addition it might be worth mentioned that egg-rolls are a far more common sight than spring rolls on the menus of such restaurants but that might be somewhat off-topic. 

We could say, as Mr. Colin McLarty writes above, that another key difference is that many consider egg-rolls as having meat (protein) with vegetables as a filling, while spring rolls only have vegetables.  And to be sure, I have never seen spring rolls anywhere in the US with any protein in the filing.  And when you order a egg-roll in a Americanized Chinese restaurant, it will contain some sport of protein.  However, one can find a curious thing in the frozen foods section of America supermarkets everywhere throughout the country.  Vegetable egg-rolls.  They are not spring rolls as they are made with the wrapper that contains egg and from the outside they look just like the egg-rolls that are sold right next to them.  But yet while you can find them in supermarkets coast-to-coast, I personally have yet to see them in any Chinese restaurant. 

Of course, all of this does not touch upon the tendency of Vietnamese restaurants in the US to call Goi cuon "spring rolls".  One can only assume such a decision is to make an ethic menu, without a typical spring roll on it, more appealing non-Asian customers.  Though some restaurants do call Goi cuon "summer rolls" instead.  And I once personally saw a menu for a Thai restaurant that had both typical egg-rolls and typical spring rolls but also offered Goi cuon as well, labeling it as the "house spring roll"!