Autocomplete Guessing?

How does the browser guess which autofill field to use? It seems to differ by URL!

by pmn4

HTML

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<div  data-hypernova-react-class="Checkout"><div class="checkout-wrapper" data-reactroot="" data-reactid="1" data-react-checksum="-2093972160"><div class="checkout__container" data-reactid="2"><!-- react-empty: 3 --><div class="column-left" data-reactid="4"><div data-reactid="5"><div class="checkout-step shipping-step" data-reactid="6"><div class="molecule-address-select molecule-address-select--no-margin" data-reactid="7"><div class="subform" data-reactid="8"><h2 class="subform-title" data-reactid="9">Add Shipping Address</h2><div class="subform-form-fields address-form-fields" data-reactid="10"><!-- react-text: 11 --><!-- /react-text --><div class="subform-field-pair" data-reactid="12"><div class="animated-input-wrapper " data-reactid="13"><input type="text" class="" tabindex="" id="firstName" name="firstName" value="" data-reactid="14"/><label for="firstName" placeholder="First Name" data-reactid="15"></label></div><div class="animated-input-wrapper " data-reactid="16"><input type="text" class="" tabindex="" id="lastName" name="lastName" value="" data-reactid="17"/><label for="lastName" placeholder="Last Name" data-reactid="18"></label></div></div><div class="animated-input-wrapper " data-reactid="19"><input type="text" class="" tabindex="" id="street1" name="street1" value="" data-reactid="20"/><label for="street1" placeholder="Address" data-reactid="21"></label></div><div class="animated-input-wrapper " data-reactid="22"><input type="text" class="" tabindex="" id="street2" name="street2" value="" data-reactid="23"/><label for="street2" placeholder="Apt / Unit / Suite (Optional)" data-reactid="24"></label></div><div class="subform-field-pair" data-reactid="25"><div class="animated-input-wrapper " data-reactid="26"><input type="text" class="" tabindex="" id="city" name="city" value="" data-reactid="27"/><label for="city" placeholder="City" data-reactid="28"></label></div><div class="animated-input-wrapper "...