Q:

WIC - Why aren't all jars of baby food WIC approved? 

I went to buy my son's (9 months) WIC baby food jars today.  Only some were WIC approved despite they were all the same stage (3), price (.97), and brand (Beechnut).  The only difference between them were the flavors.  For example, corn with squash is approved but not spinach with quinoa.  Why are some of these flavors unapproved?  

WIC-The Special Supplemental Nutrition Program for Women, Infants, and Children provides Federal grants to States for supplemental foods, health care referrals, and nutrition education for low-income pregnant, breastfeeding, and non-breastfeeding postpartum women, and to infants and children up to age five who are found to be at nutritional risk. 

http://www.fns.usda.gov/wic/women-infants-and-children-wic

A:

Here is an example of what states are allowed to approve for WIC eligibility.  States have their own standards as well. 

The following list provides the Federal requirements for WIC-eligible foods.  USDA requirements for WIC-eligible foods can be found in 7 CFR Part 246.10.  

The page discusses infant formula, exempt infant formula, milks, cheese, fruit and vegetable juices, shell eggs, canned/frozen fruits and vegetables, whole wheat bread, canned fish, and peanut butter.  
State agencies are required to authorize container sizes that provide the full maximum monthly allowance of foods without exceeding the maximum. 
Under Infant Food Fruits & Vegetables, we see that fat content, sugar content, types and amount of additives, etc.  determine eligibility. 

INFANT FOOD FRUITS and VEGETABLES 
Types/Requirements of Infant Fruits and Vegetables

Any variety of single ingredient commercial infant food fruit without added sugars, starches, or salt (e.g., sodium).  Texture may range from strained through diced.  The fruit must be listed as the first ingredient. 
Any variety of single ingredient commercial infant food vegetables without added sugars, starches, or salt (e.g., sodium).  Texture may range from strained through diced.  The vegetable must be listed as the first ingredient. 
Combinations of single ingredients (e.g., peas-carrots and apple-banana) are allowed. 

Not Allowed

Mixtures with cereal or infant food desserts (e.g., peach cobbler). 

INFANT FOOD MEAT
INFANT FOOD MEAT

Any variety of commercial infant food meat or poultry, as a single major ingredient, with added broth or gravy.  Texture may range from pureed through diced. 

Not Allowed

Added sugars or salt (e.g., sodium). 
Infant food combinations (e.g., meat and vegetables) or dinners (e.g., spaghetti and meatballs). 
Any variety of commercial infant food meat or poultry, as a single major ingredient, with added broth or gravy.  Texture may range from pureed through diced. 

Etc. 
So the difference is in additives, etc, as outlined above. 
You can read more at the link provided or at your states WIC page.