A few months ago, a coworker and I were puzzling over the index values in an application I'd built. Specifically, we were astounded to find that our request_id values were over 4,000, when we knew for a fact that there had only been a few hundred requests made so far.
Now, this was not an APEX application, but I've found that APEX, by default, is vulnerable to the same explosive growth in index values. For primary key values, which are not shown to your users (usually), this isn't really a big deal–if you declare the column to be a number without specifying a precision, Oracle guarantees support for 38 digits, so as long as you don't hit 100,000,000,000,000,000,000,000,000,000,000,000,001 you're ok1. But sometimes you may want the sequence values to be tightly packed; personally, I like a tidy database, so seeing large gaps in my primary keys irks me, even if no one else knows they're there.
So, what causes those gaps? It turns out that the standard “create sequence some_seq;” statement fills in some defaults (well, yeah, that's obvious, but bear with me). Some are what you probably want–start with 1, increment by 1–but one can be problematic. By default, Oracle caches the next twenty values of a sequence when you query its nextval; if you don't use them in that session, they're “lost”–the next nextval will give the twenty-first value. So the fix is simple: when you create a sequence, if it's only used sporadically, specify the nocache attribute.
…which brings us back to APEX. Now, I love APEX, but… When you create a sequence in the SQL Workshop, you can specify the number to cache, and leaving the field blank results in nocache being set appropriately. But. Yes, there's a but. When you're creating a table, if you let APEX generate the sequence for the primary key, it doesn't specify any attributes at all–including nocache. So, if you decide it matters, you have three options: create the sequence yourself before the table, alter the sequence after it's created, or copy the creation script from the final step of the table creation wizard, edit it, and run it manually. Personally, I'd go for one of the first two options, but hey–it's your choice.
1 1e+39 only needs one digit of precision, but (1e+39)+1 requires 39. Fun with math! Yay!