TYPO3 Core

\TYPO3\CMS\Backend\Form\AbstractNode::getValidationDataAsJsonString()

contains this code:

            if ($config['renderType'] !== 'selectTree' && $maxItems <= 1 && $minItems > 0) {
                $validationRules[] = array(
                    'type' => $type,
                    'minItems' => 1,
                    'maxItems' => 100000
                );
            } else {
                $validationRules[] = array(
                    'type' => $type,
                    'minItems' => $minItems,
                    'maxItems' => $maxItems
                );
            }

which is also called for inline records.
TCA type inline has no "renderType" definition, hence the first if branch is entered for the case when min=max=1.