The described behavior is not unique to BitGo:
wallets in Bitcoin generally track a collection of addresses. In fact, it is considered bad practice to reuse the same address (see address-reuse) for multiple payments due to privacy concerns.
As JBaczuk mentioned, BitGo uses Hierarchical Deterministic wallets. Hereby, BitGo exclusively employs 2-of-3 multisig addresses, which means that the body of addresses in a wallet are linked by all being derived from the same three master private keys and their corresponding keychains. As the name implies addresses are deterministically derived, which means that any number of addresses can be generated and any of these addresses can be recalculated with the knowledge of the three keys and the derivation path of an address.
On the V1 platform of BitGo, the first address of a wallet also seconds as the wallet identifier. On the V2 platform, wallet identifiers have a separate format from Bitcoin addresses. The wallet address is perfectly capable of receiving funds, although it may be preferable to start with the first derived address. A new address should be used for every subsequent payment or invoice after that.
Note that BitGo is only adding new features to the V2 platform, so if you are just starting to use BitGo and created a V1 Bitcoin wallet, you may want to use a V2 Bitcoin wallet instead to get access to the latest and future features.
Disclosure: I'm a software engineer at BitGo.